Even as the Web Analytics Association gets started with it's own Social Network (using KickApps) I'm very conscious of how Social Networks are ranked, and while comScore is hardly 100% accurate, I suppose it's about as close as we're going to get, today, to knowing which networks are growing the fastest, and by how much, according to TechCrunch -Social Site Rankings (October, 2007):

The way it looks to me is Facebook and Flickr are now twice as large in October 2007 as they were in October of 2006 and LinkedIn is 3 times as large. Digg has grown by a factor of 6 but most of the AOL properties have declined and MySpace hasn't grown that much, by about 25% (not that 25% is bad, but when you compare it to the grown of Facebook or Linkedin, according to this study, for whatever it's worth, is shows that MySpace is a mature property and slowly dying).