Facebook shares have fallen heavily as the first lock-up period, which stops sales by early investors, ended.
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Shares fell 4.95% to $20.15 in afternoon trade in New York, down from $20.74 on Wednesday.
About 271 million shares become eligible to be sold on Thursday, which is more than half the 421 million that have been trading since Facebook shares were floated in May at $38 each.
Since then, Facebook's share price has fallen, amid worries over its strategy.
In early trade on Thursday the shares were down by more than 6%.
Lock-ups prevent company insiders from selling their shares in a newly-floated firm, and usually start to expire 90 days from the initial public offering (IPO).
They are designed to prevent the share price from fluctuating wildly if too many investors decide to sell their shares all at once.
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