# How does Facebook know who my brother-in-law is?! **URL:** \ **Category:** Factual Questions\ **Created:** [May 6, 2016, 10:09pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108 "2016-05-06T22:09:42Z")\ **Posts on this page:** 11\ **Page:** 2
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 9, 2016, 2:14am UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/21 "2016-05-09T02:14:46Z")
OK, suppose your mom (who has no particular connection to your BiL) has Facebook, and she also uses Facebook Messenger. Facebook knows that she has a listing for “Eclectic” with a phone number, and it also knows that she’s friended with you on Facebook. There might be many other "Eclectic"s on Facebook, but given that she’s connected with both your phone number and with your Facebook account, it’s a safe bet that your phone number goes with your account. This is especially true if there are multiple people who have you in their contacts: Then Facebook can see that a whole bunch of people have both a phone contact and a Facebook friend with that name, and so be almost certain that the number and account go together. OK, so Facebook knows your phone number. Now, it can look through everyone who has Facebook Messenger, and see if there’s anyone who’s ever dialed your number, but who doesn’t already have you friended. And it then recommends that you friend each other. ---
**Author:** ![amanset](https://avatars.discourse-cdn.com/v4/letter/a/8dc957/32.png) [@amanset](https://boards.straightdope.com/u/amanset)\ **Post date:** [May 9, 2016, 9:51am UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/22 "2016-05-09T09:51:12Z")
> [@eclectic\_wench](#): > > No, I don’t have Facebook Messenger. And I don’t use Facebook on my phone. Does he? Have you ever been to the same event that was organised on Facebook? Have you ever been in a group chat with him? Frankly I wouldn’t be so quick to discount the “Friend of a Friend” thing, especially if you live in the same area. You’d be surprised. ---
**Author:** ![SpeedwayRyan](https://avatars.discourse-cdn.com/v4/letter/s/ce7236/32.png) [@SpeedwayRyan](https://boards.straightdope.com/u/SpeedwayRyan)\ **Post date:** [May 9, 2016, 12:36pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/23 "2016-05-09T12:36:25Z")
> [@jtur88](#): > > I live in an apartment complex where there is a couple with the same surname as mine. We’ve both been here for at least 6 years. It has evolved to the point where at least every week, I get junk mail addressed to THEIR name, at MY apartment number. So somehow my apartment number has been cross-referenced with their name, on the address lists of bulk mailers. Several times, I even got an e-mail asking them how they liked their new Kia and don’t forget to bring it in for servicing. (I have no car.) How did the local Kia dealer get my email address? Do you ever go to a store and they ask for your zip code? Not your full address, because they don’t want to bother you, but nobody minds just a zip code. Say you pay with a credit or debit card, so they know your name. Then they search consumer databases and find the most likely match to add that person to their mailing list…so now you are getting catalogs or whatever from that store without ever handing over your address…and they know what you bought, so they can tailor the advertising to you. Surely many of these consumer databases also have email addresses tied to names/street addresses, sold to them by someone who you gave it to willingly in the past. I am guessing that on some database, your physical address got tied to their name, probably because they saw the same last name and same street address and assumed everyone lived in one household (the apartment number was likely overlooked through either a human error or a computer snafu). ---
**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:** [May 9, 2016, 6:51pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/24 "2016-05-09T18:51:47Z")
Has anyone ever uploaded a picture of you to facebook and tagged you? Has anyone ever uploaded a picture of him and tagged him? The changes are very high that both of these have happened, and you’re not likely to know. Has anyone ever both uploaded a picture of you and a picture of him? Doesn’t have to be the same picture. Facebook’s photo processing and facial recognition are getting pretty good. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [May 9, 2016, 8:01pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/25 "2016-05-09T20:01:12Z")
> [@iamthewalrus\_3](#): > > Facebook’s photo processing and facial recognition are getting pretty good. Oh, [it’s more than just ‘pretty good’](https://www.facebook.com/help/122175507864081). And while they claim that “you can choose whether or not we suggest your name when people upload photos of you,” I’m morally certain that that algorithm runs on every image uploaded, and only the functionality to recommend tags is switched off. It can still make connections based on what they find. While you still “own” the content in the photos, you are granting Facebook royalty-free rights for use of those images at long as they are pasted on your webpage regardless of privacy settings. They assert that images are not made available for sale to third parties, but of course anyone can take public content and copy it with impunity (and normal watermarking is pathetically easy to remove). Facebook may use those images for commercial and informational purposes, including tracking and making connections between different users. Stranger ---
**Author:** ![eclectic\_wench](https://avatars.discourse-cdn.com/v4/letter/e/ac91a4/32.png) [@eclectic\_wench](https://boards.straightdope.com/u/eclectic_wench)\ **Post date:** [May 9, 2016, 10:16pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/26 "2016-05-09T22:16:30Z")
> [@Crazyhorse](#): > > Besides FB messenger, they also own Whatsapp. So if for example you both use Whatsapp and have each other in your contacts that way, and you each use those same phones to log in to FB, then FB can determine that you know each other but for reasons they cannot comprehend or accept, haven’t decided to become facebook friends yet. Like I said upthread, I have WhatsApp but he doesn’t, and I don’t have FB on my phone. I don’t even use the same email address for my phone account as I do on FB. > [@Crazyhorse](#): > > If you do actually have a mutual FB friend but both they and your brother in law have their FB preferences set not to share their friends lists, then he will appear as a suggested friend with no mutual friends to display. > > They only become a “mutual friend” after you friend your brother in law. Then you could see who the common link is. But until then, the link between them and your brother in law is not visible to you when looking at either of their profiles. > > If you look at your brother in law’s profile and can’t see who his FB friends are this is probably what happened. I can see his FB friends. > [@Chronos](#): > > OK, suppose your mom (who has no particular connection to your BiL) has Facebook, and she also uses Facebook Messenger. Facebook knows that she has a listing for “Eclectic” with a phone number, and it also knows that she’s friended with you on Facebook. There might be many other "Eclectic"s on Facebook, but given that she’s connected with both your phone number and with your Facebook account, it’s a safe bet that your phone number goes with your account. This is especially true if there are multiple people who have you in their contacts: Then Facebook can see that a whole bunch of people have both a phone contact and a Facebook friend with that name, and so be almost certain that the number and account go together. > > OK, so Facebook knows your phone number. Now, it can look through everyone who has Facebook Messenger, and see if there’s anyone who’s ever dialed your number, but who doesn’t already have you friended. And it then recommends that you friend each other. Now this makes total sense. Thank you for the explanation. I knew I was being dense 😃 That could well be what happened. > [@amanset](#): > > Does he [have Facebook Messenger/use Facebook on his phone]? I don’t know. He could well. > [@amanset](#): > > Have you ever been to the same event that was organised on Facebook? Nope. > [@amanset](#): > > Have you ever been in a group chat with him? Nope. > [@amanset](#): > > Frankly I wouldn’t be so quick to discount the “Friend of a Friend” thing, especially if you live in the same area. You’d be surprised. We don’t live in the same area. Honest to God, our lives just don’t intersect, except through his/my husband’s family members. None of whom I’m FB friends with. I mean, Ireland being what it is, we probably have a friend of a friend of a friend in common, but no more than I’d have with any random person off the street. All the other ‘People you may know’ have at least one FB friend in common with me. > [@iamthewalrus\_3](#): > > Has anyone ever uploaded a picture of you to facebook and tagged you? Has anyone ever uploaded a picture of him and tagged him? The changes are very high that both of these have happened, and you’re not likely to know. > > Has anyone ever both uploaded a picture of you and a picture of him? Doesn’t have to be the same picture. > > Facebook’s photo processing and facial recognition are getting pretty good. Interesting. I’m pretty sure I have my FB settings set to show me if anyone tags me in a picture, though. Also I’ve never posted any pics of me, so it’s not like facial recognition alone could connect me to my FB profile. ---
**Author:** ![doreen](https://avatars.discourse-cdn.com/v4/letter/d/858c86/32.png) [@doreen](https://boards.straightdope.com/u/doreen)\ **Post date:** [May 10, 2016, 2:00am UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/27 "2016-05-10T02:00:06Z")
> [@eclectic\_wench](#): > > We don’t live in the same area. Honest to God, our lives just don’t intersect, except through his/my husband’s family members. None of whom I’m FB friends with. I mean, Ireland being what it is, we probably have a friend of a friend of a friend in common, but no more than I’d have with any random person off the street. All the other ‘People you may know’ have at least one FB friend in common with me. I get lots of friend suggestions for people I don’t know and who I have nothing in common with. I just assume they are the unknown version of the “friend of friend of friends” that run into so frequently. People notice and message me something like “I didn’t know you knew Margaret. Where do you know her from?” and it turns out that my uncle’s new wife worked with the sister of someone I went to school with. But my 25 yo son’s grade school classmate’s mother doesn’t know that her husband’s sister married my high school boyfriend and I didn’t know it either till I saw a comment or photo of him on her page. Long way to say there may be connections there that you don’t know about. ---
**Author:** ![BigT](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bigt/32/12044_2.png) [@BigT](https://boards.straightdope.com/u/BigT)\ **Post date:** [May 10, 2016, 3:30pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/28 "2016-05-10T15:30:40Z")
Facebook can get your phone number from the same place anyone else would–from anything where you’ve given your name and phone number. Though I went through my recommended list, and it does look to me like the ones that don’t list any friend connections are actually a friend of a friend. (A few seem obviously connected to an overseas friend.) So I like the idea that someone in the chain doesn’t have all their friends visible. ---
**Author:** ![guizot](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/guizot/32/3636_2.png) [@guizot](https://boards.straightdope.com/u/guizot)\ **Post date:** [May 10, 2016, 5:56pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/29 "2016-05-10T17:56:03Z")
If anyone here wants to insist on all these elaborate, mysterious mechanisms, and discount the importance of the phone number, you can always do the experiment I did. (And actually continue to do.) Here’s what I did: I created a new Gmail address, and used it to create a new Facebook account, with my own name. I did NOT use a phone number to create the account. I did NOT give any personal information, except for a birthdate (which I changed from my own by one month). I did not friend anyone, or even invite any friend requests, nor did I request any friends. All I did was post a few photos (without people, but with some comments). I liked (or “joined”) ONLY ONE promotional web page, that of the Carnaval of Barranquilla, Colombia. Basically, except for that, the account is a zombie. No one has posted on it, and I haven’t used the account to post anywhere, either. I started to use the Gmail account, but ONLY with colleagues in Colombia, and I put their email addresses in the Google contact list for those people–but NO phone numbers. I waited for about three months, but didn’t get any “You may know this person,” announcements. Then, about a month ago, I asked a friend–who doesn’t have a Facebook account, and who doesn’t have Whatsapp–if I could experiment with her phone number, by adding it to this “ghost” account. (She lives in San Francisco.) Once I put in that phone number, suddenly there were DOZENS of “You may know suggestions.” Mostly people from San Francisco. Almost all of them people I don’t know, but some I know, just through this friend who let me borrow her number. Not one of the suggestions is a person from Colombia, even though the emails in the Gmail contacts account is only Colombians. When I showed her the list, my friend in San Francisco also said, “HEY!!! How do they know that I know that person–I have NO communication with that person,” etc. about several of the people. So this shows that "You may know … " suggestions can _ **come entirely** _ _ **without** _ all of the complicated avenues of connecting people from the friends lists, or looking at your Facebook activities. IT CAN COME ENTIRELY FROM THE PHONE NUMBER. Of course, this doesn’t mean that Facebook doesn’t probably use those other methods, but, it does show that they don’t have to. My next step is to get my friend to start a WhatsApp account on her phone, and selectively add people to it, to see if those people show up on my ghost Facebook page. Also, another plan I have (when I get some time) is to start using the ghost account in selective ways, bit by bit, to see how various activities on Facebook might change the list of “You may know…” suggestions. For example, I plan on friending one person, a total stranger in another country, to see what happens. Then I’m going to start adding phone numbers to the Gmail contact list, of the people in Colombia, to see what happens then. In other words, step by step I want to see how all of these other uses will affect the suggestions. ---
**Author:** ![Omar\_Little](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/omar_little/32/269_2.png) [@Omar\_Little](https://boards.straightdope.com/u/Omar_Little)\ **Post date:** [May 10, 2016, 6:43pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/30 "2016-05-10T18:43:40Z")
Have you ever searched for him on FB? Maybe he searched for you on FB! That’s enough right there. ---
**Author:** ![Mama\_Zappa](https://avatars.discourse-cdn.com/v4/letter/m/71e660/32.png) [@Mama\_Zappa](https://boards.straightdope.com/u/Mama_Zappa)\ **Post date:** [May 10, 2016, 9:32pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/31 "2016-05-10T21:32:33Z")
If anyone who has ever emailed you both let Facebook at their email address book, that’s one thing I can think of, and it’s one of the reasons I never let Facebook etc. “help” me find friends. I got a friend request from a woman who worked for us as a nanny 18 years ago. We had no email contact with her and neither of us uses the same email as we did back then. We have no phone number for her (landline or cell). Facebook does not have my home phone number. We have no friends in common. Yet, I cropped up on her list of suggested friends. Bizarro. I actually ignored the friend request - we’ve no desire to maintain contact. [Previous page](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108.md?page=1)