When Facebook insiders, employees can sell stock

Up to 1.56 billion more shares could flood the stock market — nearly four times the 421 million shares that had been trading since Facebook's initial public offering in May. The 1.56 billion figure includes shares released from lock-up already.

On Aug. 16, a lock-up period expired for 271 million shares held by early investors and directors who had participated in the IPO, though CEO Mark Zuckerberg was excluded for unspecified reasons.

On Oct. 29, the lock-up ended for 234 million shares and stock options held by employees as of Oct. 15, excluding Zuckerberg. Oct. 31 was the first trading day since then because of stock market closures resulting from Superstorm Sandy.

On Wednesday, another 852 million shares and stock options became eligible. 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 until at least next September.

Here's the schedule for remaining lock-up expirations, as reported by Facebook Inc. in regulatory filings:

— 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.