Is the great Facebook stock sale over?SThrough the golden heart of every world-changing startup pulses an avaricious get-rich-quick scheme. Larry Page and Sergey Brin, the billionaire-boy cofounders of Google, established this doing-well-by-doing-good myth. But Mark Zuckerberg hasn't been able to make the same magic happen for his employees. In his efforts to make good by them, he may end up quashing a nascent market in Facebook shares.It's not for lack of trying. Silicon Valley's stock-options millionaire make money by getting the right to buy shares at a low price and selling them for a higher one. Facebook's soaring valuation — Microsoft invested $240 million for a tiny stake, in a deal which valued all of Facebook at $15 billion — threatened to undo that equation. How is Facebook supposed to soar past $15 billion in value? So Zuckerberg & Co. turned to issuing restricted stock units, or RSUs, instead. (Restricted stock units are common at large companies like Google and Microsoft, but unusual for a company Facebook's size and age.) The restricted-stock plan has created a new complication: Once it has more than 500 RSU holders, SEC regulations may require Facebook to start publishing its financials, even if it doesn't conduct an initial public offering. Facebook's revenues still aren't pretty enough for public exposure. Facebook's lawyers have sought, and obtained, an exemption. Part of the argument they made is that issuing RSUs won't create a market in Facebook shares. Facebook, unusually for most of Silicon Valley's private companies, has not had many restrictions on what employees and other shareholders could do with the shares they own. Most have rules that force shareholders to offer shares to the company first — a right of first refusal — or outright prohibitions on unauthorized sales. But the letter Facebook sent to the SEC says that even when the stock units convert to common shares, they have limits on their sale: "... the Plan has been structured to preclude any trading of RSUs or any interest therein from developing." Even if Facebook permits an employee convert their stock units to shares and sell them, the company can then prevent the buyer from selling. Employees at Facebook — especially the early ones, whose holdings are now substantial — have been agitating for some time to sell their shares, and there are still, even with the public markets taking a beating, interested buyers. Zuckerberg finally bowed to this pressure and set up a program, now underway, to let employees cash out up to $900,000 in shares. (Note the symbolism of the figure: No one will become a millionaire.) But that may be it. If Facebook extends its stock-sale restrictions to common shares, not just the restricted-stock units, both employees and the investors so eager to snap up their shares will be stuck, until Facebook sells out or goes public. Zuckerberg has made it clear he thinks both of those events are far off — and the 24-year-old CEO still owns 27 percent of the company and more or less controls the board. It's a dicey gamble. The prospect of selling Facebook shares privately must surely have attracted some employees who counted on a relatively quick cash-out. But shutting down the prospect of further stock sales will make sure the Facebookers who remain will be more committed to the company for the long haul. Zuckerberg doesn't have much choice. As long as Facebook employees can find buyers for their shares, they'll be tempted to leave rather than stay at a company going through a tumultous adolescence. Already, the company has had far more turnover, from bottom to top, than Google did. Not a single high-ranking exec left Google for the first six years of its existence. Facebook has lost three of its four cofounders, and numerous people underneath them, from former COO Owen Van Natta on down. No wonder Zuckerberg wants to slam the exit door closed.