The stock sank Tuesday after regulatory filings showed that a Facebook executive is unloading more shares even as CEO Mark Zuckerberg was pledging to keep all of his stock for at least a year.

Last month, Facebook’s early investors and some company directors became eligible to sell stock they own in the social networking company. As of Aug. 16, 271 million shares were released for sale.

Up to 1.5 billion more shares could flood the stock market — about 3.5 times the 421 million shares that had been trading since Facebook’s IPO. The company’s stock has fallen since the Aug. 16 expiration of the first lock-up period, when company executives and employees are allowed to start selling the shares they bought or were given at the time of the IPO.

In an effort to get past the impact of sales by those who bought or were given shares at the time of the IPO, a regulatory filing by the Menlo Park, Calif.-based company revealed that Facebook is moving up the expiration date of a post-IPO ban on stock sales by employees by two weeks.

Many of Facebook’s rank-and-file employees can start selling stock they own on Oct. 29 instead of the earlier date of Nov. 14. Facebook says it’s withholding about 101 million shares from its employee stock units to cover taxes they will incur at a rate of about 45%. It will then pay the tax obligations, currently estimated at around $1.9 billion, cash and credit.

Facebook’s stock (FB) fell to $17.55 in Tuesday trading, its lowest point ever, and closed down 33 cents at $17.73, its lowest close since its May 18 IPO. The stock was down 1.8% for the day, and it has tumbled more than 50% from its IPO price of $38.

Filings made public Tuesday, covering sales since last Thursday, showed that Facebook co-founder Dustin Moskovitz had shed another 450,000 of the social networking company’s shares, for a take of about $8.3 million.

Moskovitz has been selling 150,000 shares a day and disclosing the sales every three days. Including the most recent sales, he still holds about 132 million shares. He sold shares at prices ranging from $17.56 to $19.42.

Here’s the schedule of the number of shares of to be released for sale, as reported by Facebook in regulatory filings:

— Oct. 29: 234 million shares and stock options held by employees as of Oct. 15, excluding Zuckerberg.

— Nov. 14: 777 million shares and stock options. Zuckerberg had been eligible to sell his shares at this date, but Facebook has changed that given that the CEO had no plans to do so.

— Dec. 14: 156 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.