http://mashable.com/2013/04/12/social-media-demographic-breakdown/
Some interesting stats from Pew, via Mashable. Make sure to read the article in the link above, but also study this chart from Docstoc. I’m intrigued by the “some college” education level as the heaviest users. And about the level of use of LinkedIn. I believe there is more to be mined here than what you see below in the aggregate.