Thanks to Megan for finding these statistics while doing research for our paper for the Identity and Identification in a Networked World: A Multidisciplinary
Graduate Student Symposium.

Nothing too surprising here. Myspace was number one, of course, with a 367% one year growth and the highest user retention rate. YouTube was number five with 12.5 million unigue users in about nine months. According to the Nielsen/NetRatings' latest figures, the Top 10 social-networking sites now reach a whopping 45% of active Web users, Together, the ten sites have grown 47% in the past year (from 46.8 million unique visitors in April 2005 to 68.8 million last month).