1) All of the people I have legitimated as Friends. 2) Screeds and screeds of data about the quality of all my connections with those friends (have I hid them from my news-feed? how much do I interact with their posts? how much do I interact with them? talk to them? share with them? stalk them? how much time do I spend in interactions with them? how do the patterns of my interactions change with them (mouse-click patterns, scroll patterns, typing patterns)? do I use different words (different emotions/more formal/informal language/different function word patterns?) 3) People I've stalked but haven't friended 4) Brands, companies, causes, ideas, things from history, all sorts of random shit I've liked (along with similar data to 2)) 5) What sorts of things I click on, how I leave the site and where I go. And certainly more.
Etc., etc., etc. Facebook didn't manifest my entire social life. They just built a platform which allowed me to socialise orders of magnitude more efficiently than before, a platform of great benefit to me, and one into which I've poured vast quantities of data (and the amount, quality and variety of the data is only set to increase as we open up more input modalities - eg eye-tracking, gesture-tracking, and biometrics). the data is all there. All facebook is doing is cobbling together some of the most very basic tools one might use to make sense of a subset of that data.