This is the most comprehensive compilation of stats I’ve seen, nice work Jeremiah.

http://www.web-strategist.com/blog/2008/11/19/social-networks-site-usage-visitors-members-page-views-and-engagement-by-the-numbers-in-2008/

“No single metric is a good indicator, you have to evaluate the usage from multiple dimensions, so you also have to factor in what are users doing, time on site, interaction, and of course, did they end up buying, recommending products, or improving their lives.”

One of the interesting links is this, Forrester have put out this article on how to compare social media tools, “Technographics” http://www.forrester.com/Groundswell/profile_tool.html