Marketwatch have just released this “The social networking
phenomenon also continued its stratospheric ascent, as MySpace.com reached new
heights with 50 million visitors in May [2006]” that is virtually the equivalent of the whole UK population visiting the site in one month!

Youtube is ramping up as well “YouTube.com nearly doubled its
traffic from April, reaching 12.6 million visitors.” It will be interesting to see when Beet.tv starts to appear in the ratings.

Bonus link: You tube dominates video search

More with nice graphs here