Beginning Thursday, Facebook's early investors and a few directors will be able to sell stock they own in the company as a lock-up period barring them from doing so begins to expire.

Thursday marks 90 days after Facebook's initial public offering. That's when early investors who were selling stockholders in the IPO get to sell their shares, according to regulatory filings. Other shareholders, such as many Facebook employees, will be able to sell beginning in October. The last lockup deadline expires next May, a year after Facebook's IPO.

Facebook's hotly anticipated IPO landed with a thud on May 18. After pricing at $38, the stock closed at $38.23 on its first trading day and has not seen that price since.

Shares in premarket trading fell nearly 2 percent, or 40 cents, to $20.80. If that holds, the stock will open Thursday at a 45 percent discount to the IPO price just three months ago.

In all, 271 million shares will become eligible for sale this week, according to Facebook's regulatory filings. Firms ranging from Accel Partners to Goldman Sachs, Zynga CEO Mark Pincus and Facebook board members James Breyer and Peter Thiel and LinkedIn Corp. Chairman Reid Hoffman are among those free to sell stock they own. Microsoft Corp., an early Facebook investor, is another one, though it's unlikely to because of partnerships it has with Facebook.

There are currently 421 million Facebook shares available for public trading.