Facebook is not a listed company, but in United States, privately held shares are traded over the counter (OTC). Over the last few years two online platforms have gained credibility as a safe medium for transaction of private shares -- Second Market and Shares Post.
Early investors and employees fear that Facebook's market valuation will not keep pace with its growth, perhaps this is why they intend to unload their stock while they can.
Previous attempt to unload the shares were at a price which valued the company at $90 billion, making it more valuable compared to Time Warner and News Corp put together. But as buyers stayed away the price was subsequently brought down.