800M Facebook shares could be sold Wednesday

(CNN) - Hundreds of millions of Facebook shares could hit the market Wednesday, when early employees and investors get a chance to sell about 800 million shares.

Facebook's May 18 initial public offering included a "lockup" agreement that restricted some shareholders from selling for a certain period of time. This is the third of five lockup expirations, and by far the largest.

The lockups are meant to help boost stock price by keeping stock supply scarce but it hasn't worked. Facebook shares have lost nearly half their value since debuting at $38.