facebook asks if you'd like to let them look at your contacts to find friends.
basically others have done that so facebook already knows Joe, Mike, and Sara are friend with [email protected] so whenever you sign up, facebook already know who's already associated with your email, or your phone#, or whatever else.
yeah, linkedin does that shit but without any of the algorithms involved so every meow and then I'm asked if I want to connect with some random person that I told a futon to on craigslist a dozen years ago