Who writes Wikipedia?

A very interesting article appeared on Aaron Swartz's blog (thanks to Daring Fireball for a pointer) where he talks about the breakdown of those creating content on Wikipedia and about how (contrary to comments by Wikipedia founder Jimbo Wales) the site appears to be constructed by a very large number of users and edited by a core group of 500-1000 (Wales contends that it is written by the core group of 500-1000).