How Facebook Figures Out Everyone You've Ever Met

Having issued this warning, and having acknowledged that people in your address book may not necessarily want to be connected to you, Facebook will then do exactly what it warned you not to do. If you agree to share your contacts, every piece of contact data you possess will go to Facebook, and the network will then use it to try to search for connections between everyone you know, no matter how slightly—and you won’t see it happen.

Facebook doesn’t like, and doesn’t use, the term “shadow profiles.” It doesn’t like the term because it sounds like Facebook creates hidden profiles for people who haven’t joined the network, which Facebook says it doesn’t do. The existence of shadow contact information came to light in 2013 after Facebook admitted it had discovered and fixed “a bug.” The bug was that when a user downloaded their Facebook file, it included not just their friends’ visible contact information, but also their friends’ shadow contact information.