# 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:** 20\ **Page:** 1
**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 6, 2016, 10:09pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/1 "2016-05-06T22:09:42Z")
OK, so Facebook is freaking me out. The last couple of weeks, Facebook’s ‘people you may know’ list for me includes my brother-in-law. I have no idea how they could possibly have made the connection. We don’t have each other’s email address. We have zero Facebook friends in common (everyone else on that list has at least a couple of Facebook friends in common with me). We have zero real-life friends in common. We have zero interests in common. We’ve never commented on the same post or tagged each other in a post. We have a last name in common, but we share it with like half of Ireland. How did Facebook make the connection?! ---
**Author:** ![zombywoof](https://avatars.discourse-cdn.com/v4/letter/z/9e8a1a/32.png) [@zombywoof](https://boards.straightdope.com/u/zombywoof)\ **Post date:** [May 6, 2016, 10:20pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/2 "2016-05-06T22:20:01Z")
Previous threads: [thread](http://boards.straightdope.com/sdmb/showthread.php?t=523932) [thread](http://boards.straightdope.com/sdmb/showthread.php?t=631391) [thread](http://boards.straightdope.com/sdmb/showthread.php?t=581120) [thread](http://boards.straightdope.com/sdmb/showthread.php?t=557971) ---
**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 6, 2016, 10:59pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/3 "2016-05-06T22:59:47Z")
[_The Atlantic_: “Facebook Is Making a Map of Everyone in the World” (And they’re using a brand new A.I. technique and a whole lot of computing power to do it.)](http://www.theatlantic.com/technology/archive/2016/02/facebook-makes-a-new-map-of-everyone-in-the-world/470487/) This isn’t a direct answer to your question, but the fact is that Facebook is trying to use large data filtering and pattern recognition methods in order to make connections and find data about people who are not nominally connected or may not even be regular users of Facebook as a social media platform. The focus is supposedly on non-internet-connected people in the developing world, but the same algorithms let them find patterns and track connections on existing “clients”. Yes, this is basically the plot of _Captain America: The Winder Soldier_, minus the flying helicarriers in “continuous suborbital flight” constantly surveilling everyone. Oh, wait, [yes, they are working on that](http://www.wired.com/2016/01/facebook-zuckerberg-internet-org/). Add one crazy scientist, a secret cabal of lawful evil maniacs, a genetically engineered supersoldier, a former KGB assissin, and a hero-worshiping former Pararescue operator with a flying suit, and you have an incredible game of _Fiasco_ or a terrifying international conspiracy. Take your pick. 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 7, 2016, 1:38pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/4 "2016-05-07T13:38:03Z")
> [@zombywoof](#): > > Previous threads: > > [thread](http://boards.straightdope.com/sdmb/showthread.php?t=523932) > > [thread](http://boards.straightdope.com/sdmb/showthread.php?t=631391) > > [thread](http://boards.straightdope.com/sdmb/showthread.php?t=581120) > > [thread](http://boards.straightdope.com/sdmb/showthread.php?t=557971) Almost everything in those threads deals with possibilities that I’ve already eliminated. We don’t have each other’s email address, and in any case the email I use for Facebook isn’t one I’ve ever used for anything else or given to anyone. We have no friends in common and I’d bet a fair amount of money that we don’t have any friends of friends in common. Honestly, our lives just don’t touch. We get on fine, but we don’t intersect. The only possibility from those threads that makes sense is that he searched for me and that triggered Facebook’s algorithm. From those threads, Facebook denies that it works that way, but who knows. Another possibility I thought of is that he’s using Facebook off a smartphone, gave it access to his contacts, and it picked up on my phone number (which I think he does have). I don’t remember giving Facebook a phone number and wouldn’t have done it unless it was required for registration, but if I did give them one, it would’ve been the one he has. ---
**Author:** ![OldGuy](https://avatars.discourse-cdn.com/v4/letter/o/3bc359/32.png) [@OldGuy](https://boards.straightdope.com/u/OldGuy)\ **Post date:** [May 7, 2016, 1:53pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/5 "2016-05-07T13:53:20Z")
> [@eclectic\_wench](#): > > We have no friends in common and I’d bet a fair amount of money that we don’t have any friends of friends in common. Honestly, our lives just don’t touch. We get on fine, but we don’t intersect. I don’t quite understand. He’s your brother-in-law so either he’s your spouse’s brother or your sibling’s spouse. That’s the obvious connection. Even if it’s not direct whichever that connection is probably share numerous other contacts. ---
**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 7, 2016, 1:56pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/6 "2016-05-07T13:56:24Z")
He’s my husband’s brother. He’s not Facebook friends with my husband. They don’t have any Facebook friends in common. They don’t have any real-life friends in common. My husband isn’t Facebook friends with any of their relatives. ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [May 7, 2016, 2:00pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/7 "2016-05-07T14:00:35Z")
> [@eclectic\_wench](#): > > Almost everything in those threads deals with possibilities that I’ve already eliminated. We don’t have each other’s email address, and in any case the email I use for Facebook isn’t one I’ve ever used for anything else or given to anyone. We have no friends in common and I’d bet a fair amount of money that we don’t have any friends of friends in common. Honestly, our lives just don’t touch. We get on fine, but we don’t intersect. A van drove by your house and detected your wifi signal and cellphone signal, also your husband’s cellphone signal. Combined with your name from a public database (property tax records are one), we now have something useful. A similar scrap of data may exist about your BIL. Did your husband ever visit his brother while his phone was in his pocket? Bingo! A connection can be made. Sure, there are many false positives from this method, but that doesn’t seem to concern those who compile all of this data. Sometimes they get lucky. If I order a security report on myself, which I did once just for kicks, I found an association with an address where I never lived, and is only a mail drop anyway, too small for an apartment. I’m sure the connection was made from a fraudulent change of address filed by a crook who picked up a credit card carbon from a trash can (We know that happened.) The problem was resolved, but the connection had been made, and _this was 40 years ago._ It still shows on some records. Now a connection can be made between me and anyone else who had a mail-drop account at the same place over 40 years. The name of the game is to make as many connections as possible and accuracy be damned. ---
**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 7, 2016, 2:12pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/8 "2016-05-07T14:12:48Z")
> [@eclectic\_wench](#): > > Another possibility I thought of is that he’s using Facebook off a smartphone, gave it access to his contacts, and it picked up on my phone number (which I think he does have). I don’t remember giving Facebook a phone number and wouldn’t have done it unless it was required for registration, but if I did give them one, it would’ve been the one he has. Have you installed Facebook messenger? Then OF COURSE it has your phone number. Every time these threads come up (i.e., “How does Facebook know I know \_\_\_\_!!!:eek::eek:”) it seems like the last thing people consider is telephone numbers in contact list, but that’s how Facebook does MOST of this “Do you know . . .” thing. That’s why you can get total strangers when they ask, “Do you know …?” They are people who have assumed the phone number of someone you know, who gave up that phone number. ---
**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 7, 2016, 2:16pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/9 "2016-05-07T14:16:05Z")
No, I don’t have Facebook Messenger. And I don’t use Facebook on my phone. ---
**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 7, 2016, 2:27pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/10 "2016-05-07T14:27:23Z")
> [@eclectic\_wench](#): > > No, I don’t have Facebook Messenger. And I don’t use Facebook on my phone. If you have a contact list on your computer, then it can make the same connection. Somehow Facebook probably knows your phone number. ---
**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 7, 2016, 2:33pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/11 "2016-05-07T14:33:06Z")
No contact list on my computer. I have WhatsApp on my phone - since they’re owned by Facebook, they might share data - but he apparently doesn’t. I’m leaning towards the ‘he searched my name on Facebook’ explanation. ---
**Author:** ![Nansbread1](https://avatars.discourse-cdn.com/v4/letter/n/6a8cbe/32.png) [@Nansbread1](https://boards.straightdope.com/u/Nansbread1)\ **Post date:** [May 7, 2016, 4:11pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/12 "2016-05-07T16:11:40Z")
Your brother in law found you on facebook. He then made a family tree on fb and put you in it. You only found out when FB told you.:smack: ---
**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 7, 2016, 4:22pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/13 "2016-05-07T16:22:32Z")
Does anyone else you know at all have Facebook Messenger? Because if they do, then Facebook can learn your phone number from _their_ contacts list. ---
**Author:** ![chappachula](https://avatars.discourse-cdn.com/v4/letter/c/d2c977/32.png) [@chappachula](https://boards.straightdope.com/u/chappachula)\ **Post date:** [May 8, 2016, 4:47am UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/14 "2016-05-08T04:47:24Z")
> [@](#): > > how does Facebook know who my brother-in-law is? Facebook _doesn’t_ know who your brother-in-law is. They know you exist, and they know he exists. They don’t know what relationship you have with him. He could be a blood relative, or he could be your plumber. Or your psychiatrist. But they found a traceable connection, through several degrees of separation, using data that you don’t know exists. (And, like you, I feel creeped out by that. That’s why I have never gone anywhere near Facebook. ) [/hijack][/imho] ---
**Author:** ![jtur88](https://avatars.discourse-cdn.com/v4/letter/j/e9c0ed/32.png) [@jtur88](https://boards.straightdope.com/u/jtur88)\ **Post date:** [May 8, 2016, 3:04pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/15 "2016-05-08T15:04:01Z")
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? Given sufficient time, the number of false positives will multiply to the point at which everyone is presumed to be everyone else, and the spammers won’t care because the cost of contacting 7 billion real people and unknown billions of imaginary ones is the same as contacting one. I rarely use Facebook for anything, but I have an account. Once I scrolled down through the “Do you know” list, and gave up out of boredom after about 500 and there was no sign of the bottom of the page, the sidebar indicator just kept repositioning itself. Every once in a while I’d recognize an unusual surname of someone I went to high school with 60 years ago. Or there would be a cluser of names in an alphabet I don’t even recognize. ---
**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 8, 2016, 11:06pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/16 "2016-05-08T23:06:26Z")
> [@chappachula](#): > > Facebook _doesn’t_ know who your brother-in-law is. > > They know you exist, and they know he exists. > They don’t know what relationship you have with him. He could be a blood relative, or he could be your plumber. Or your psychiatrist. I know that. The wording of the title is crappy, but yeah, I know. > [@chappachula](#): > > But they found a traceable connection, through several degrees of separation, using data that you don’t know exists. Well, yes, clearly. What I want to know is what kind of data could possibly have led them to that connection. > [@Chronos](#): > > Does anyone else you know at all have Facebook Messenger? Because if they do, then Facebook can learn your phone number from _their_ contacts list. See, here’s where I get dense. Explain like I’m 5: how would they figure out my number from someone else’s phone contacts? So let’s say my brother-in-law uses Facebook Messenger. They mine his contacts list and find my number. How do they know that, out of the dozens of Eclectic Wenches on Facebook, that phone number belongs to me? And if he’s not using Facebook Messenger, if they got my number out of someone else’s contact list, then even if they somehow knew which Eclectic Wench it belonged to, I’m back to how on earth would they connect me to my brother-in-law? ---
**Author:** ![Crazyhorse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crazyhorse/32/31_2.png) [@Crazyhorse](https://boards.straightdope.com/u/Crazyhorse)\ **Post date:** [May 8, 2016, 11:38pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/17 "2016-05-08T23:38:18Z")
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. But the situation you describe also happens all the time for a much simpler reason. 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. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [May 8, 2016, 11:50pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/18 "2016-05-08T23:50:07Z")
> [@eclectic\_wench](#): > > So let’s say my brother-in-law uses Facebook Messenger. They mine his contacts list and find my number. How do they know that, out of the dozens of Eclectic Wenches on Facebook, that phone number belongs to me? How do you know that Facebook hasn’t put him on the “people you may know list” for every one of those Eclectic Wenches? Sounds like something they would do, and it does say “may know,” not “definitely know.” ---
**Author:** ![lance\_strongarm](https://avatars.discourse-cdn.com/v4/letter/l/87869e/32.png) [@lance\_strongarm](https://boards.straightdope.com/u/lance_strongarm)\ **Post date:** [May 8, 2016, 11:55pm UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/19 "2016-05-08T23:55:59Z")
> [@eclectic\_wench](#): > > I’d bet a fair amount of money that we don’t have any friends of friends in common. He’s your stinkin’ brother-in-law. I doubt you can say that with confidence. ---
**Author:** ![Crazyhorse](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crazyhorse/32/31_2.png) [@Crazyhorse](https://boards.straightdope.com/u/Crazyhorse)\ **Post date:** [May 9, 2016, 12:11am UTC](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108/20 "2016-05-09T00:11:17Z")
> [@Gary\_T](#): > > How do you know that Facebook hasn’t put him on the “people you may know list” for every one of those Eclectic Wenches? Sounds like something they would do, and it does say “may know,” not “definitely know.” They could but they wouldn’t need to. The apps already know their unique phone numbers. They wouldn’t need to look at names if they found a number in another phone’s contacts and that number has ever been used to log in to FB, messenger, Whatsapp, etc. But the more likely scenario is what I posted above. If you see a suggested friend that has no mutual friends it’s often because they, and a mutual friend that you really do have in common, both have their friends lists set to private. Facebook knows that connection is there but it can’t show you how it knows in the friend suggestion because you don’t have access to that information due to the two other account’s privacy settings. [Next page](https://boards.straightdope.com/t/how-does-facebook-know-who-my-brother-in-law-is/754108.md?page=2)