content: Facebook maintains shadow profiles, creating one for you even if you never signed up. Every page that has a Like button and every site which participates in a data-sharing agreement with Facebook provides them with valuable information about who you are, who your friends are, and what interests you have. This goes double, of course, anytime Facebook obtains your friends' contact lists, photos, etc. Your place in the social graph is easily constructed even without you voluntarily providing any of your personal information. By the way, it's not like Facebook is the only company who's figured this out. Analytics is a multi-billion dollar industry, and for every major company who you know is doing this, there are dozens you've never heard of that probably have even more of your information.