Understanding what happens behind sites and where people come from is a constant challenge of emarketeers.  I just found this very nice post from a blog called socialfishing which identified a great tool to get detailed demographic about big sites, including social networks, search engines and other sites that qualify. 

It’s called quantcast and it seems to deliver some great insights.  Check out the summary of soime of the key networks by socialfishing:

“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.”

Worth a look.