Facebook likes big numbers - it now has more than 500 million users, each one of whom can have as many as 5,000 friends. Yet as a privately held company, its ownership base must remain small, or it will have to disclose publicly its financial results.

A surging shadow market in the privately held shares of Facebook is making such restraint difficult and could spur the company to go public - even as its executives try to tamp down speculation about an initial public offering - much as similar pressure helped push Microsoft and Google toward their own initial public offerings.

The frenzied trading in Facebook, as well as in Twitter, Zynga and LinkedIn, has caught the eye of the Securities and Exchange Commission. The New York Times DealBook first reported on Tuesday that the agency had asked for information about trading in all four companies.

While it is unclear what exactly the S.E.C. is focusing on, legal experts say that one clear area of inquiry relates to a federal law that establishes a limit for private companies of fewer than 500 shareholders. Once a company has 500 shareholders, it must register its private shares with the S.E.C. and publicly disclose its financial results.

Facebook is well aware of this issue. In 2008, the S.E.C. allowed Facebook to issue restricted stock to employees without having to register the securities, a move that would have required the company to publicly disclose financial information.