Without much surprise there has been significant movement within some of the social networks over the course of the last year. Brian Chappell from Ignite Social Media has put together a comprehensive report based on data from all major social networks existing on the web.
S. Caldeira, D. Coutinho, and G. Teixeira. Abstract Book of the XXIII IUPAP International Conference on Statistical Physics, Genova, Italy, (9-13 July 2007)