One of the challenges with "social networking" sites is that most are more correctly "social linking" sites.
...they are all like bad parties where everyone is gathered in small circles with their backs to anyone new. One of the benefits of a good host/hostess (other than attracting an interesting crowd) is his/her ability to introduce individuals to other individuals who are likely to share some kind of interest. To my knowledge, no social networking site is particularly good at making introductions and most do not even try.
Right on. We train networkweavers to make useful & actionable introductions.