The Internet is a vortex of personal data — much of it not accurate, and some of it creepily so. As a result, we all encounter ads, recommendations and personalizations whose provenance we can’t possibly know. Why did Facebook recommend that a psychiatrist’s patients “friend” each other? How does Google know where I live? If you send us your own data mysteries, we’ll attempt to sort them out. Use the form at the bottom of this post to submit your questions.

Hi, Intersect,

I deactivated my Facebook account about a year ago, but recently reactivated it. When I did, I was surprised to see Facebook recommend a number of “friends” I met in that off year — including one guy I see at the dog park sometimes and have no other connections with. How does Facebook know I met these people when I wasn’t even using Facebook? It seems really weird.



Hey, Jason,

This might seem a little odd, but there’s actually a pretty obvious explanation. When you reactivated Facebook you probably also downloaded the app to your phone again. And I’d guess you clicked “yes” when asked, at/near start-up, if you wanted to “Find Your Friends.”

You have to manually turn on this feature, but a lot of people do it without realizing what it means. Once you’ve opted in, Facebook continuously culls information about your friends from your phone, including “names and any nicknames; contact photo; phone numbers and other contact or related information you may have added like relation or profession.”

The network then uses that information to recommend new friends. So if you added an acquaintance’s number to your address book within the past year, there’s a good chance you’ll get a prompt encouraging you to friend them.

What if you didn’t turn on “Find Your Friends,” though? (This is the situation that tends to freak people out.) The feature can still recommend friends to you, based on the information it has collected from others. Say, for instance, that Facebook has begun recommending co-workers who joined your company in the past year — that makes sense, because you share a “current workplace” and many mutual friends. Or maybe you and dog-park dude share a dog walker, and she has opted into uploading her contacts. The two of you then coexist in her address book and in a location you’ve presumably put on your profile, which — to Facebook’s eyes, at least — might mean you’re friend-able.


If you want to turn off continuous uploading, you can do that through the Facebook app. There is, however, no foolproof way to stop the “weird” recommendations sourced from other people’s contacts. It’s also worth noting that Facebook updates this stuff all the time, so it’s difficult to nail down. Try brainstorming commonalities with your new friends and let us know what you’ve found.

Good luck,

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