On Thursday, Facebook's early investors and some directors became eligible to sell stock they own in the social networking company. Others will have similar rights in the coming months.

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.

It's not yet known whether any of those investors had sold any shares. Facebook's stock fell Thursday, but it could have reflected investors' anticipation of such a move.

Here's the schedule, as reported by Facebook Inc. in a regulatory filing:

- Thursday: 271 million shares held by early investors and directors who had participated in the IPO, though CEO Mark Zuckerberg is excluded for unspecified reasons.

- Unspecified date between Oct. 15 and Nov. 13: 243 million shares and stock options held by directors and former or current employees, excluding Zuckerberg.

- Nov. 14: 1.22 billion shares and stock options, about a third of which is controlled by Zuckerberg.

- Dec. 14: 149 million shares held by early investors and others who participated in IPO, except Zuckerberg.

- May 18, 2013: 47 million shares held by the Russian Internet company Mail.ru Group and DST Global, both of which made early investments in Facebook.