The agency has sent requests for information to several participants in the buying and selling of stock in those four companies, the newspaper reports, citing two unnamed sources.
Most of the sellers are former employees and early-stage venture capitalists, while the buyers have been mostly wealthy speculators -- individuals with at least $1 million in net worth or institutions with over $100 million -- although some Wall Street brokerages have been forming investment pools for the stocks.
In the past, companies like these would have already floated their shares publicly, but these days they are waiting longer to take the IPO step. Instead, private exchanges such as SharesPost and SecondMarket are serving as intermediaries between buyers and sellers, with steady increases in user base, number of companies traded and volume.
But while investors are betting on these companies to have sky-high valuations when they do go public, for now, they don't have to disclose their financial information, making the numbers more speculative and investments that much riskier.
It is unclear exactly what has piqued the SEC's interest, the The New York Times says, but experts told the newspaper it could relate to the number of shareholders in these companies. Once that number reaches 500 shareholders, companies are required to disclose their financial results to the public. It's possible the pooled vehicles push that number above the 499 threshold, which could mean that sometime soon, we'll all get to a look into the financial guts of these social behemoths.