A really nice presentation (224 slides but moves right along) from Paul Adams, who works with the Google User Experience team, on the true nature of social networks. The main point is that we do not have one amorphous group of "friends" or contacts, but rather, several distinct groups. We belong not to one network but to many.
If you don't have time for the 224 slides in this presentation, here's the actual data Paul presents. The ideas and numbers presented are also, in my not so humble opinion, relevant to adjunct organizing efforts.