The company started by facilitating the liquidation of assets produced during bankruptcy, but SecondMarket is getting a reputation as a venue where employees and other holders of private stock in whitehot, pre-I.P.O. tech companies like Facebook. The private stock exchange still constitutes a minor part of its business, but it's growing. Venture capital funds were the most active buyers, making up more than 40 percent of complete trades, the company said, and almost 20 percent of buyers were "high net-worth investors."

The company is also pushing to get traditional money managers using the service.

SecondMarket is one of four firms offering private share trading, according to SecondMarket. The others are: Xpert Financial Inc., which has not yet started trading; SharesPost Inc., an online bulletin board for buyers and sellers; and Nasdaq OMX’s PORTAL Alliance LLC, which trades only 144A or private company share transactions among qualified institutional buyers with more than $100 million in assets.

SecondMarket received an inquiry letter recently from the SEC, which was looking into whether the market for shares in private companies, namely Facebook, was skirting investor rules requiring disclosures--so far, the company has escaped any formal investigations or allegations of wrongdoing.