KEY POINTS
A news ticker announces the falling share price of Facebook in Times Square on August 16, 2012 in New York City. Shares of Facebook fell today on the NASDAQ stock exchange on the first day insiders were allowed to sell their shares.

With its third-heaviest volume ever, Facebook shares closed below $20 for the first time on Thursday, as its initial lockup period expired. The intensified selling pressure came on the first day that some Facebook insiders could begin selling their stock.

Thursday’s six percent sell-off capped a very rough first three months for the social-network giant's stock. Its price has fallen 48 percent since it went public at $38 on May 18. During this period, Facebook has underperformed every single other Dow Industrials, S&P 500 and Nasdaq 100 stock.