How can social networks make money

With social networks now big business, what can the companies do to turn free services into hard cash?

Source:
http://news.bbc.co.uk/2/hi/programmes/click_online/9457946.stm

One Call To Action

"Decide an important action + encourage fans to do the action." There are three phases according to Vinson: Attract fans from social networks a.k.a. "outposts." Engage them with a "compelling fan experience." Sell through simple calls to action.

Have one concept per page���one call to action.

Source:
http://virtualmusic.tv/2010/07/nms_one-call-to-action/