Battle of the social sites

This is a great chart that shows that Facebook has more users than MySpace and Twitter.
Although I am on a quest to find out more about the audience demographics and how do people use each of these sites.
Who actually tweets on Twitter - is it mainly celebrities and marketers?
Is MySpace just teenagers into bands and music?
Does anyone use Facebook apps anymore and if so who?
Would be great to get an accurate unbiased overview of who, why and what.

(Image from http://www.penn-olson.com/2009/10/08/a-gorgeous-chart-facebook-twitter-myspace-stats/)