Facebook shares closed below $20 today.
The first time it crossed this threshold—nearly half the price at which it debuted on the stock market—was in August.
The stock rallied after CEO Mark Zuckerberg spoke at a conference in San Francisco in September. Among other remarks, he explained the company's missteps in mobile and said that the company was committed to building a strong, valuable business.
It fell again in October, then rallied after Facebook announced third-quarter earnings that showed a reacceleration in growth.
Since then, post-IPO lockups on insiders have started expiring, releasing more shares on the market and generating downward pressure on the stock price.
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