Quantcast tracks demographic information on the web.
So here's Facebook. 52 million US people. It skews slightly younger, female, educated (college), with children. (Click on the "demographics" tab for the summary).
Here's LinkedIn. 9.3 million US people. It skews slightly older, male, highly educated (grad school), no children, more affluent.
Here's Twitter. 2.4 million US people. Skews more female, more Gen-X, less affluent, educated (college and grad school).
Here's Blogger.com. 23.5 million people. Skews male, younger, middle income, post grad.
Try it for yourself - plug in Ning.com, or CollectiveX.com, or Amazon, YouTube, Flickr or any other social site you can think of. Think about your target audiences and your stakeholders, your members - where might they be hanging out?
Link to original post