When a firm undertakes an IPO, insiders (owners and venture capitalists, among others) agree to retain their shares (i.e., lockup) for a period of time, typically six months. When the lockup period expires, it is customary to see a large block of shares flood the market, having an adverse effect on the stock's price. Facebook just hit its lockup expiration. See article here, NY Times.
Dolvin, Steven D., "Facebook IPO Lockup Expiration" (2012). All Chapters. 34.