How Facebook figures out everyone you’ve ever met

Behind the Facebook profile you’ve built for yourself is another one, a shadow profile, built from the inboxes and smartphones of other Facebook users. Contact information you’ve never given the network gets associated with your account, making it easier for Facebook to more completely map your social connections.

…Facebook isn’t scanning the work email of the attorney above. But it likely has her work email address on file, even if she never gave it to Facebook herself. If anyone who has the lawyer’s address in their contacts has chosen to share it with Facebook, the company can link her to anyone else who has it, such as the defense counsel in one of her cases.

How Facebook Figures Out Everyone You’ve Ever Met

In real life, in the natural course of conversation, it is not uncommon to talk about a person you may know. You meet someone and say, “I’m from Sarasota,” and they say, “Oh, I have a grandparent in Sarasota,” and they tell you where they live and their name, and you may or may not recognize them.