While Mark Zuckerberg may be in no hurry to take Facebook public, the same apparently can’t be said for his employees. The emergence of private stock exchanges, which link holders of private-company stock with buyers, has led to a booming trade in tech companies that have yet to have an IPO, like Facebook, Twitter, Lined In, and Zynga Inc. The value of trades in private-company stock has more than doubled in 2010 to $4.9 billion. Some companies, like Facebook, have stopped issuing stock options to new employees in an attempt to crack down on trading, though to little avail. As trading increases, so does the risk that private companies will run afoul of regulation, and that buyers, who have little insight into the workings the companies, will misstep.