Social Graph: Social networks that map your existing offline relationships, and are merely a website that provides online tools to enhance and foster these connections (i.e. Facebook, LinkedIn).
Interest Graph: Social networks that are built around specific topics of interest, and are formed because individuals are lacking such relationships in their offline lives and thus aggregate into communities online (i.e. Twitter, Citizun).
Good design adds value of some kind, gives meaning, and, not incidentally, can be sheer pleasure to behold; it respects the viewer’s sensibilities and rewards the entrepreneur. - Paul Rand