THE US Securities and Exchange Commission is looking into the booming trade in the privately held shares of popular social networking behemoths such as Facebook, LinkedIn, Twitter and Zynga, according to reports in The New York Times and elsewhere.

Citing unnamed people with knowledge of the matter, The Times said the SEC has sent "information requests" to several participants in the trading of these four companies' shares.

Shares of privately held companies can be traded on private stock exchanges such as SecondMarket, based in New York, and SharesPost, based in San Bruno, California. The shares are generally sold by former employees or early investors in these firms. Only institutional investors or high net-worth individuals - those worth over US$1 million - can buy the shares.

A big reason the SEC may be curious about the trading of these popular private startups' shares is because once a company hits 500 shareholders, it must make certain financial data public, even if it hasn't filed for an initial public offering. Facebook has been trying to put off reaching this threshold. For example, it has barred current staff from selling their shares.