It's déjà vu all over again for Facebook. On Wednesday, the company faces yet another day when millions of shares could hit the market -- and this may the biggest potential stock dump by far.

Early employees and investors will get their first chance to sell about 773 million shares, as well as another 31 million restricted stock units owned by employees who joined the company prior to 2011. Like many initial public offerings, Facebook's May 18 debut included a "lockup" agreement that requires some shareholders from selling for a certain period.