The Securities and Exchange Commission is starting to examine stock trades for the hot social-networking companies Facebook, Twitter, Zynga and Linked In.

Despite the fact that all of these companies are privately held, the SEC is asking their private stockholders about their stock trades, which are facilitated by several private exchanges created to match buyers with sellers.

The reason? While the volume of these trades is still low, it’s apparently growing — so much so that some Wall Street brokerage firms such as GreenCrest Capital and Felix Investments have begun forming investment pools to buy these companies’ shares, raising “Facebook funds” or “Twitter funds,” that pool clients’ funds into investment vehicles to acquire blocks of social networking company stocks.

And if the number of shareholders tops 500, the SEC would require them to disclose their financial results to the public.

Interest in social-network investing is driven not only by the firms' incredible success, but by the longer length of time such companies take to go public.

“A decade ago, these companies would be public by now,” David Weir, the chief executive of SharesPost, an online marketplace for private investments started last year that now has 39,000 registered members, told The New York Times

“Investors can now buy into these businesses and sellers can exit their already valuable stakes.”

The worth of these private companies has soared along with interest in their shares:

SharesPost reports that Facebook is now trading at an implied valuation of $42.4 billion, according to SharesPost, more three times its value earlier this year, while Twitter is worth $3.6 billion, more than doubling during the last several months.

Indeed, SecondMarket, the leading trading exchange handling transactions in these securities, is reported to be expecting to execute about $400 million in trades involving about 40 private companies this year — almost four times last year’s trades.

However, razzle-dazzle aside, experts say investors are assuming substantial risk along with their social-networking shares: Facebook, for example, doesn't disclose its financial results and Twitter generates only minimal revenue.

And, although the trading in these companies has increased during the last year, the market remains illiquid, capped by the amount of stock for sale.

The Washington Post, which reported that Time magazine named Facebook founder Mark Zuckerberg “Person of the Year,” also reported that Zuckerberg pledged $100 million to the Newark, New Jersey, school system and has joined fellow entrepreneurs Bill Gates and Warren Buffett in pledging to give away most of his wealth.