Facebook warns users to be judicious about using all this data. “You may have business or personal contacts in your phone,” the Learn More screen admonishes the reader. “Please only send friend requests to people you know personally who would welcome the invite.”
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.
It’s actually not that difficult when billions of people upload their contact info to Facebook to draw these links.