# How does Facebook know I know these people? **URL:** \ **Category:** Factual Questions\ **Created:** [July 9, 2009, 3:46pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462 "2009-07-09T15:46:32Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Brandus](https://avatars.discourse-cdn.com/v4/letter/b/898d66/32.png) [@Brandus](https://boards.straightdope.com/u/Brandus)\ **Post date:** [July 9, 2009, 8:58pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/21 "2009-07-09T20:58:15Z")
I am pretty sure that recently they have started to suggest people that have searched for you. Recently their guesses have been surprisingly accurate, including a “not close” friend who I haven’t talked to in years, have never emailed, and have no friends in common. I assumed he wondered “I wonder whats up with that guy?” and searched for me. ---
**Author:** ![MeanOldLady](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/meanoldlady/32/10737_2.png) [@MeanOldLady](https://boards.straightdope.com/u/MeanOldLady)\ **Post date:** [July 9, 2009, 9:21pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/22 "2009-07-09T21:21:18Z")
> [@HorseloverFat](#): > > Your friends are probably doing this: > > [How Facebook Uses Your "Skipped" Webmail Contacts](http://blogoscoped.com/archive/2009-06-12-n15.html) > > and importing your email address. On top of that it will do stuff like match high schools, interests, locales, etc. And what if they’re not, hmm? What if it suggested someone who hasn’t even visited the website [facebook.com](http://facebook.com) in four years? What if the location in said person’s profile is horribly outdated and lists one you’ve never lived in, and certainly never indicated you did on your facebook page, hmm? _I demand answers!_ Is Facebook reading this too? We need a paranoia emoticon. ---
**Author:** ![Telemark](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/telemark/32/372_2.png) [@Telemark](https://boards.straightdope.com/u/Telemark)\ **Post date:** [July 9, 2009, 9:43pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/23 "2009-07-09T21:43:32Z")
As others have said, Facebook can’t read your contacts unless you give them permission and login. They can’t read your email. They can’t check your website for visitors. They just use an algorithm that is good at matching 2nd and 3rd order connections, the same way that LinkedIn and other social networking sites make suggestions. It’s a complex computational problem, but smart people have been working on it. Plus, you ignore all the complete misses that Facebook suggests so you think it’s doing something really clever. Nah, it’s just throwing out a bunch of potential names and hopes that some look familiar. ---
**Author:** ![Patty\_O\_Furniture](https://avatars.discourse-cdn.com/v4/letter/p/96bed5/32.png) [@Patty\_O\_Furniture](https://boards.straightdope.com/u/Patty_O_Furniture)\ **Post date:** [July 9, 2009, 9:54pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/24 "2009-07-09T21:54:01Z")
From their [privacy policy](http://www.facebook.com/policy.php): > [@](#): > > **The Information We Collect** > When you visit Facebook you provide us with two types of information: personal information you knowingly choose to disclose that is collected by us and Web Site use information collected by us as you interact with our Web Site. > > When you register with Facebook, you provide us with certain personal information, such as your name, your email address, your telephone number, your address, your gender, schools attended and any other personal or preference information that you provide to us. > > When you enter Facebook, we collect your browser type and IP address. This information is gathered for all Facebook visitors. In addition, we store certain information from your browser using “cookies.” > > _\_ > > When you use Facebook, you may set up your personal profile, form relationships, send messages, perform searches and queries, form groups, set up events, add applications, and transmit information through various channels. We collect this information so that we can provide you the service and offer personalized features. In most cases, we retain it so that, for instance, you can return to view prior messages you have sent or easily see your friend list. When you update information, we usually keep a backup copy of the prior version for a reasonable period of time to enable reversion to the prior version of that information. > > When you register with Facebook, you create your own profile and privacy settings. Your profile information, as well as your name, email and photo, are displayed to people in the networks specified in your privacy settings to enable you to connect with people on Facebook. We may occasionally use your name and email address to send you notifications regarding new services offered by Facebook that we think you may find valuable. > > Profile information is used by Facebook primarily to be presented back to and edited by you when you access the service and to be presented to others permitted to view that information by your privacy settings. In some cases where your privacy settings permit it (e.g., posting to your wall), other Facebook users may be able to supplement your profile. > > Profile information you submit to Facebook will be available to users of Facebook who belong to at least one of the networks you allow to access the information through your privacy settings (e.g., school, geography, friends of friends). Your name, network names, and profile picture thumbnail will be available in search results across the Facebook network and those limited pieces of information may be made available to third party search engines. This is primarily so your friends can find you and send a friend request. People who see your name in searches, however, will not be able to access your profile information unless they have a relationship to you (friend, friend of friend, member of your networks, etc.) that allows such access based on your privacy settings. > > We may use information about you that we collect from other sources, including but not limited to newspapers and Internet sources such as blogs, instant messaging services, Facebook Platform developers and other users of Facebook, to supplement your profile. Where such information is used, we generally allow you to specify in your privacy settings that you do not want this to be done or to take other actions that limit the connection of this information to your profile (e.g., removing photo tag links). I snipped out a bunch of n/a stuff. A few months ago a good friend of mine uploaded an old picture to his profile. This was taken at a party a few years ago. He tagged me and a bunch of other people, none of who I really knew. Over the next few days I received friend suggestions from FB. So just because I was tagged in the same photo as several other people, FB evidently thought that we all might want to become friends. As it turned out, I did not have any desire to know these people and I simply ignored the request. End of paranoia. ---
**Author:** ![MeanOldLady](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/meanoldlady/32/10737_2.png) [@MeanOldLady](https://boards.straightdope.com/u/MeanOldLady)\ **Post date:** [July 9, 2009, 10:01pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/25 "2009-07-09T22:01:41Z")
Right. That makes sense if you have friends in common, friends of friends in common, attended the same school, work for the same company, or list anything in common on your pages. I don’t blink when they suggest someone I went to junior high school with or some such. If a guy only has one Facebook friend, and that friend is not a friend of mine on Facebook, isn’t a friend of any of my friends on Facebook, has never even heard of anyone I’ve ever know on Facebook or otherwise, and is clear across the damn country in a network I’ve never been a part of, how are they connecting me to the guy? The guy has not revised any of his information in years, so his current information is from a million years ago, linked to a location I’ve never been to. That is the fanciest logarithm I’ve ever seen. Why would they think I know this random person from Illinois? Facebook says: _We may use information about you that we collect from other sources, including but not limited to newspapers and Internet sources such as blogs, instant messaging services, Facebook Platform developers and other users of Facebook, to supplement your profile._ They’re supplementing profiles with information they collect from “other sources” such as newspapers and blogs? ---
**Author:** ![PatriotGrrrl](https://avatars.discourse-cdn.com/v4/letter/p/9dc877/32.png) [@PatriotGrrrl](https://boards.straightdope.com/u/PatriotGrrrl)\ **Post date:** [July 9, 2009, 10:06pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/26 "2009-07-09T22:06:56Z")
> [@MeanOldLady](#): > > And then I saw the spouse, who has about zero internet presence, _the day after_ I sent an e-mail to his personal account (which I’ll guess he signed up with) instead of his e-mail address, with no explanation for why they suggested him… and then I thought, they monkeys be readin’ my e-mails! Could you define the term “his personal account” as opposed to “his e-mail address”? If you sent a message to his facebook account (as opposed to an email address) it’s no great stretch to think you might want to friend him. ---
**Author:** ![MeanOldLady](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/meanoldlady/32/10737_2.png) [@MeanOldLady](https://boards.straightdope.com/u/MeanOldLady)\ **Post date:** [July 9, 2009, 10:10pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/27 "2009-07-09T22:10:54Z")
I’ve never sent him a message via Facebook. He doesn’t use Facebook. He signed up because someone told him to, added her as a friend, and never went back. I mean I sent an e-mail to his personal gmail account (the one he used to sign up with), when I usually send e-mails to his work account. The next thing, _poof_ you may know this guy. ---
**Author:** ![Rigamarole](https://avatars.discourse-cdn.com/v4/letter/r/77aa72/32.png) [@Rigamarole](https://boards.straightdope.com/u/Rigamarole)\ **Post date:** [July 9, 2009, 10:39pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/28 "2009-07-09T22:39:54Z")
> [@MeanOldLady](#): > > I’ve never sent him a message via Facebook. He doesn’t use Facebook. He signed up because someone told him to, added her as a friend, and never went back. I mean I sent an e-mail to his personal gmail account (the one he used to sign up with), when I usually send e-mails to his work account. The next thing, _poof_ you may know this guy. Well, they were right. You do know him. Time to stop worrying and love the bomb… ---
**Author:** ![iamthewalrus\_3](https://avatars.discourse-cdn.com/v4/letter/i/258eb7/32.png) [@iamthewalrus\_3](https://boards.straightdope.com/u/iamthewalrus_3)\ **Post date:** [July 9, 2009, 11:07pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/29 "2009-07-09T23:07:47Z")
I suggest an experiment. Create two fake facebook profiles with no shared networks and with a free webmail account each. Have one of them share their email and have an email message sent between them. Wait a few days. Try having one of them search for the other one, but not send a friend request. This is probably against the facebook TOS, but I doubt they’d catch you. ---
**Author:** ![MarcinCiez](https://avatars.discourse-cdn.com/v4/letter/m/b38774/32.png) [@MarcinCiez](https://boards.straightdope.com/u/MarcinCiez)\ **Post date:** [July 9, 2009, 11:15pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/30 "2009-07-09T23:15:41Z")
> [@Frylock](#): > > How on earth could Facebook connect me with these guys? ---- maybe ---- Google your name and some of the results will contain links to The SDMB. You have your email listed here in the contact details… and so do some of your SDMB friends… well… it linked those 2 emails together and whala! So for me it’s not hard to see that FB has a robot crawling the www and looking for links. ---
**Author:** ![ultrafilter](https://avatars.discourse-cdn.com/v4/letter/u/3d9bf3/32.png) [@ultrafilter](https://boards.straightdope.com/u/ultrafilter)\ **Post date:** [July 9, 2009, 11:58pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/31 "2009-07-09T23:58:41Z")
> [@MarcinCiez](#): > > ---- maybe ---- > > Google your name and some of the results will contain links to The SDMB. You have your email listed here in the contact details… and so do some of your SDMB friends… well… it linked those 2 emails together and whala! > > So for me it’s not hard to see that FB has a robot crawling the www and looking for links. Designing a program to do this would be expensive and not particularly beneficial, so I kinda doubt that it exists. ---
**Author:** ![kushiel](https://avatars.discourse-cdn.com/v4/letter/k/bbce88/32.png) [@kushiel](https://boards.straightdope.com/u/kushiel)\ **Post date:** [July 10, 2009, 12:27am UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/32 "2009-07-10T00:27:42Z")
> [@MeanOldLady](#): > > I’ve never sent him a message via Facebook. He doesn’t use Facebook. He signed up because someone told him to, added her as a friend, and never went back. I mean I sent an e-mail to his personal gmail account (the one he used to sign up with), when I usually send e-mails to his work account. The next thing, _poof_ you may know this guy. Hmmmm, random brainstorming here. So he only has one friend? Maybe she was talking to another friend that knew your husband, and found he was married to **MeanOldLady**. Having been reminded of your husband, she goes to his Facebook page. Then she thinks, hey, maybe I should look up his wife! And Facebook thinks that if she went to your husband’s page and searched for you, maybe you’d know him too! ---
**Author:** ![otorophile](https://avatars.discourse-cdn.com/v4/letter/o/3ab097/32.png) [@otorophile](https://boards.straightdope.com/u/otorophile)\ **Post date:** [July 10, 2009, 1:05am UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/33 "2009-07-10T01:05:27Z")
> [@iamthewalrus\_3](#): > > I suggest an experiment. Create two fake facebook profiles with no shared networks and with a free webmail account each. Have one of them share their email and have an email message sent between them. > > Wait a few days. Try having one of them search for the other one, but not send a friend request. > > This is probably against the facebook TOS, but I doubt they’d catch you. Ok, I’ll try it ---
**Author:** ![fiddlesticks](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fiddlesticks/32/18129_2.png) [@fiddlesticks](https://boards.straightdope.com/u/fiddlesticks)\ **Post date:** [July 10, 2009, 4:10am UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/34 "2009-07-10T04:10:28Z")
Facebook picked up one of my Flickr contacts, probably because I use the MyFlickr app…I found that interesting… I have a bunch of other mysterious “People You May Know” lately who I have no idea how we were connected, like a girl from Argentina, for instance… ---
**Author:** ![Hrududu](https://avatars.discourse-cdn.com/v4/letter/h/3ec8ea/32.png) [@Hrududu](https://boards.straightdope.com/u/Hrududu)\ **Post date:** [July 10, 2009, 11:28am UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/35 "2009-07-10T11:28:01Z")
Facebook recently suggested my aunt. We dont have the same surname or any friends in common. They could only have linked us through my email account. I once gave them access, maybe 18 months ago (before I had her email address) so it looks like they still have access, and not just from that one off. ---
**Author:** ![Fetchund](https://avatars.discourse-cdn.com/v4/letter/f/b2d939/32.png) [@Fetchund](https://boards.straightdope.com/u/Fetchund)\ **Post date:** [July 10, 2009, 12:07pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/36 "2009-07-10T12:07:37Z")
They will suggest people living in the same area with the same last name, too. ---
**Author:** ![MeanOldLady](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/meanoldlady/32/10737_2.png) [@MeanOldLady](https://boards.straightdope.com/u/MeanOldLady)\ **Post date:** [July 10, 2009, 1:30pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/37 "2009-07-10T13:30:11Z")
> [@Rigamarole](#): > > Well, they were right. You do know him. Time to stop worrying and love the bomb… Shut up, shut up, shut up! > [@kushiel](#): > > Hmmmm, random brainstorming here. So he only has one friend? Maybe she was talking to another friend that knew your husband, and found he was married to **MeanOldLady**. Having been reminded of your husband, she goes to his Facebook page. Then she thinks, hey, maybe I should look up his wife! And Facebook thinks that if she went to your husband’s page and searched for you, maybe you’d know him too! _glares_ Maybe. I like my paranoia idea better. > [@Fetchund](#): > > They will suggest people living in the same area with the same last name, too. In my particular case, we don’t have the same last name, and according to our FB pages, live states apart. Hey **walrus** , I was just thinking of a little experiment myself. I set up a dummy account, and started e-mailing with it. The dummy lives in Aberdeen, SD and has no interests. I’m not even going to have one account search for the other, I’m just going to see if e-mailing alone will make FB suggest the dummy to me. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [July 10, 2009, 1:39pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/38 "2009-07-10T13:39:57Z")
> [@iamthewalrus\_3](#): > > I suggest an experiment. Create two fake facebook profiles with no shared networks and with a free webmail account each. Have one of them share their email and have an email message sent between them. > > Wait a few days. Try having one of them search for the other one, but not send a friend request. > > This is probably against the facebook TOS, but I doubt they’d catch you. Done. Nothing happened. Did this months ago, BTW. ---
**Author:** ![JFLuvly](https://avatars.discourse-cdn.com/v4/letter/j/ac91a4/32.png) [@JFLuvly](https://boards.straightdope.com/u/JFLuvly)\ **Post date:** [July 10, 2009, 2:30pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/39 "2009-07-10T14:30:00Z")
This thread caught my attention so I started looking at the ads on the right side of my facebook, and they seem to be directed towards stuff that i have looked at on the internet the last few days…interesting??? Examples… Round of golf cost at a local course = Golf ads Try to find out what I need to cross the Can-Am border on vacation = Cam-Am Immigration ads Searching google for software to get info from a corrupt harddrive = Lost data ads Definately going to keep an eye on this stuff. ---
**Author:** ![Blaster\_Master](https://avatars.discourse-cdn.com/v4/letter/b/cab0a1/32.png) [@Blaster\_Master](https://boards.straightdope.com/u/Blaster_Master)\ **Post date:** [July 10, 2009, 2:54pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462/40 "2009-07-10T14:54:50Z")
I’m going to take a bit of a guess here that it is actually a pretty complicated implementation of a 6 degrees of separation thing, except a bit more complicated. In fact, networking, especially social networking, has been a very hot topic for research in algorithms in recent years. Considering that this is what Facebook is designed for, I wouldn’t see it as surprising that they might have some pretty complicated implementations. For instance, you have the obvious example where two people share mutual friends, there’s a certain probability that they probably know eachother. However, the thing to keep in mind here is that having some number of friends in common isn’t going to translate linearly to a probability. Say, for instance, me and Aaron each know Beth, Chad, and David all of whom are fairly normal users, then it’s fairly likely that Aaron and I know eachother. Now consider a scenario where Aaron and I share three friends, but these friends are, say, Barrack Obama, Cameron Diaz, and David Beckham (sticking with my alphabetic naming convention); if they’re on Facebook (I have no idea), chances are they’d have plenty more connections than everyone else, so it’s much less likely that Aaron and I know eachother based on those connections. Now extrapolate this out another level, where Aaron and I don’t have any friends in common, but many of our friends have some level of established connections, like say I know Beth, Chad, and David who are all mutually friends with Eric, Frank, and George who are friends with Aaron, then it’s not unlikely that Aaron and I know eachother. They also have other information at their disposal, like what high school or college you went to, where you live, where you work, your age, etc. and all of that can be used in there as well. The quote upthread also says it make look at cookies, so it’s not unlikely that it could see that two people two people visit a site that tends to have a tighter connection (like the SDMB vs. [ESPN.com](http://ESPN.com)), and it would also use that to weight common connections more. To **walrus** and **Philster** , the reason that experiment didn’t work is probably because that’s not how people generally interact. I’d think that it really can’t work unless you actually made some connections and worked them. That is, I’d guess that not all friends are created equally. If I were on facebook and interacted heavily with some friends, but not with others, I’d think those links would be weighted more than the other ones, so even if I have fewer connections to someone through those links than with someone else that I don’t interact with much, it may guess that I’m more likely to know the former. If someone just creates a Facebook account, but never makes friends, or has very few friends, and never interacts with them, I’d suspect that it won’t do a very good job of connecting them to anyone. [Previous page](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462.md?page=1) [Next page](https://boards.straightdope.com/t/how-does-facebook-know-i-know-these-people/502462.md?page=3)