As of today here are some of the numbers:
Facebook 350 million users (stats vary, some say 400 million)
MySpace 130 million users (I find it interesting that MySpace is not being taken seriously lately but it still has this many users!!)
Twitter 75 million users
Classmates 50 million users
LinkedIn 35 million users
Flickr 20 million users
Source: wikipedia (thank you Margaret Donnelly)