Erick Schonfield has a great post on Techcrunch abou the size and growth of the key social network sites in the US. His table is reproduced below, but please go and read his commentary - at the comments at the bottom of his pot..
The key points to remember are:
1) this is just US, other networks are stronger elsewhere
2) although in % terms FaceBook grew four times as much as MySpace, the absolute increase in numbers is similar
3) there are other ways of doing the numbers, and they will give different results.