MENLO PARK, Calif. — ­Facebook’s­­ early investors and a handful of directors will become eligible on Thursday to sell stock they own in the social networking company. It marks the beginning of a time-honored process for public companies, one that will give many Facebook staffers the same right to sell their shares this fall.

It’s conceivable none of them will sell. But if they do, up to 1.91 billion more shares could flood the stock market — more than four times the 421 million shares that have been trading since Facebook’s initial public offering in May.

So-called lock-up periods, which prevent insiders from unloading shares too close to an IPO, generally start to expire 90 days after a stock debuts.

Lock-ups are designed to prevent a stock from experiencing the volatility that might occur if too many shareholders decide to sell a newly traded stock all at once. The phasing in of various shareholders allows early owners to shed their stock and make way for new investors, said Peter Zaleski, a professor of economics at Villanova School of Business.

But there’s risk, too. If too many people sell, Facebook Inc.’s stock price could decline.

That’s a problem the company can’t afford. On Tuesday, the stock closed at $20.38, down 46 percent from its initial public offering price of $38.

In all, 271 million shares will become eligible this week.

Wedbush analyst Michael Pachter said it’s unlikely top executives will sell as soon as they can; it would look bad.

About 243 million more Facebook shares and stock options will enter the public stock market between Oct. 15 and Nov. 13.

Facebook’s stock has not hit its IPO price since its first day of trading. The company’s market value has plummeted from $104 billion to $59.1 billion.