Trading stocks in privately held social-network websites like Facebook? The Securities and Exchange Commission may ask you some questions.

The market regulator says it's looking into people who are buying and selling stocks in privately-held social-media networks Facebook, Twitter, Linked In and Zynga, sources tell the New York Times.

New exchanges are popping to handle such trades.

Sellers include former employees or venture capitalists involved with the companies in question, while buyers are mainly are wealthy speculators.

Services that match buyers and sellers report trading volume that has room to really grow.

"We are serving a growing need," says David Weir, chief executive of SharesPost, an online marketplace for private investments.

“A decade ago, these companies would be public by now. Investors can now buy into these businesses and sellers can exit their already valuable stakes.”

Other firms are popping up to let investors trade shares on social-network sites as well.

SecondMarket should execute about $400 million in trades of 40 private companies in 2010, four times as many in 2009, the company says.

Social-networking sites are surging in value and are affecting valuation models.

"SharesPost reports that the average valuation on Facebook shares traded on its a private exchange has risen almost 25 percent to more than $56 billion in the past month," according to the Silicon Valley/San Jose Business Journal.

"SecondMarket, said it has seen a 12 percent rise in Facebook's average valuation."