Forrester's Jeremiah Owyang is trying to aggregate site usage stats for social networks -- useful stuff.
He mentions that his post from back in January on MySpace and Facebook stats gets considerable traffic. I've noticed (actually, Scott noticed) that a post I did back in December 2006 on the same subject is still one of the most highly-viewed pages on the Lithosphere. I guess people are just hungry for numbers.
In fact, that old post of mine is still pretty useful. It's useful not for the stats it includes -- social networks have grown just a little bit, if you haven't noticed -- but for the reference it makes to the Wikipedia page that tracks such things. And that Wikipedia page is useful not because its stats are particularly up to date, but because the references contain some good sources for current info on social net usage.
Here's an example: the reference for Twitter is a site called Twitdir. This site will tell you faithfully how many registered users there are on Twitter. (Today, the answer is 3,328,420.) I say faithfully because Twitdir uses Twitter's API to compile its directory.