Doc Searls unpacks a lot in this description of the difference between Twitter and Facebook.
…the online world has both social media and social habitats. They are different, even when they overlap. Twitter is a social medium. Facebook is a social habitat. Twitter is a new breed of Web site/service that grew out of blogging. Facebook is a walled garden: a place you have to go to be social in the ways it facilitates and permits. In this respect Facebook is AOL 2.0. By calling both “social media” we blur distinctions that are necessary for making sense of highly varied progress (or movement in less positive directions) in the online world.