Rumors are floating around today that the US Securities and Exchange Commission (SEC) is conducting a probe into the private trading going on in companies like Facebook and Twitter. Twitter isn’t traded publicly, and the SEC wants to know more about the private deals that go on behind closed doors.

There is no official word from the SEC yet, but the New York Times reports that sources close to the situation say that requests have been sent to individuals who hold private Twitter stock.

Altogether, the SEC is rumored to be looking at Facebook, Twitter, Zynga and LinkedIn.

Trading privately means that companies can keep the majority of their financial information secret, and it protects them from the general rules of being a public company. This might be the reason why the SEC has decided to take a harder look at these four behemoths of new media, although there is no official statement available.

The New York Times explains that the “secondary markets” of private trading in businesses like Twitter usually materialize when founding employees and early investors decide to sell off their stock. And buyers are usually wealthy investors looking to jump on the next big thing in tech.

The New York Times also speculates that one of the reasons the SEC may be looking in to Twitter, Facebook and the others is to find out just how many private investors each company has. If any of them have over 500, that company would be required to release their financial information to the public.