UPDATE 1-Facebook shares fall as lock-up period expires

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Oct 31 (Reuters) - Facebook Inc shares fell 4 percent to $21.07 in busy early trade on Wednesday morning on the company's approval to let employees sell some stock.

The world's largest social network waived a provision that prevented employees from selling shares until Nov. 14. As a result, Facebook staffers were able to sell their vested shares on Oct. 29.

However, because the markets were closed on Monday and Tuesday due to powerful storm Sandy, Wednesday was the first trading day. About 234 million shares held by employees are eligible for sale in the public market.

Facebook suffered a painful public debut earlier this year, as investors worried about the company's ability to keep up its revenue growth and the large pool of additional shares in the lock-up that are now hitting the market.

Wall Street also has cast a gimlet eye on Facebook and its ability to attract mobile revenue as more people turn to smartphones and tablet devices to access the web.

Last week, Facebook said it grew its mobile advertising revenue at a faster than expected pace, totaling $150 million in the third quarter. Estimates had pegged mobile revenue at $40 million to $50 million in the second quarter.

(Reporting By Jennifer Saba; Editing by Gerald E. McCormick and Claudia Parsons)