Where do social networking sites get their revenue from?

I was just really wondering where social networking sites such as Myspace and Facebook.com get their revenue from. this question came to me while on Cnbc.com and i saw that microsoft was considering purchasing facebook.com for $6 billion. and it said that facebook was worth well over 2 billion. it is free to join these sites so where does the revenue come from?

Where do social networking sites get their revenue from?
advertising on the website. pay-per-click, consumer surveys, and commercials. they can also sell demographic information. if 95% of the 15 year olds on the site do a certain thing, that info is valuable and can be sold.
Reply:More social networks are moving to subscriber because they offer more value
Reply:Advertising! Companies will pay big bucks when the audience is 150 million people.