Facebook insiders could sell 773 million shares Wednesday

The largest set of Facebook stock shares available since May, when the company went public, could hit the market on Wednesday as the social network's employees are given the green light to sell off 773 million shares.

Wednesday marks the expiration of another "lockup" period, which means early investors and Facebook employees, including those who joined before 2011, will be able to sell off 773 million common shares, plus 31 million in restricted stock units.

The previous lockup expired at the end of October, and that fresh infusion of stock into the open market left the share price down 3.4 percent at $21.12 per share at closing.

Since then, the stock has fallen some more, opening at $19.15 this morning.

Wednesday would also be the day CEO Mark Zuckerberg could sell 504 million of his shares, bumping the total amount of shares available to 1.3 billion, but he's already said he won't be selling until at least September 2013.

His top executives may not be following suit. Among those who recently converted their restricted stock into shares that they can sell are COO Sheryl Sandberg and CFO David Ebersman.

The next expiration date is December 14, when more than 155.9 million shares may free up. This is when stockholders who sold into the IPO -- including big names like early backers Peter Thiel and Mark Pincus and co-founder Dustin Moskovitz -- are free to sell their stock.

After December, the next date to look for is May 18, when more than 47.3 million shares can be sold. These shares include those held by investors like Mail.ru Group Limited and DST Global Limited.