Facebook Inc. (Nasdaq: FB) is freeing up 271.1 million of its shares today, boosting by 60 percent the number that could be traded and adding to concerns that have weighed on the stock since the company’s initial public offering.

Early Facebook investors such as DST Global Ltd., Goldman Sachs Group Inc., Elevation Partners and Accel Partners get a green light today to start selling part of their holdings, Menlo Park, California-based Facebook has said in filings. That’s after the lifting of restrictions designed to prevent a flood of shares immediately after an IPO.

The prospect of more stock sales means Facebook will need to work even harder to persuade investors that it deserves a higher valuation, compared with earnings, than all but two of its closest competitors including Google Inc. The shares freed up today make up only 14 percent of the 1.91 billion that will be available for sales in the coming nine months.

“Buckle your seatbelts for the next couple of months until they make it through all these shares coming unlocked,” said Tom Forte, an analyst at Telsey Advisory Group in New York.

The shares subject to today’s lock-up expiry are worth about $5.75 billion, based on yesterday’s closing price, and represent almost six times the average daily trading volume in Facebook stock. The restriction is being lifted for early investors, excluding Facebook Chief Executive Officer Mark Zuckerberg, who sold part of their holdings in the IPO.

Facebook closed at $21.20 in New York Wednesday. The shares haven’t closed above the IPO price of $38 since the first day of trading. Ashley Zandy, a spokeswoman for Facebook, declined to comment.

Why Sell?

Facebook, worth $51.2 billion, has lost about $40 billion in market value since the IPO, making it the worst performer among all large IPOs on record, according to data compiled by Bloomberg.

Still, the decline has been steep enough to make some investors consider holding onto shares instead of selling as quickly as they can, said Mark Harding, an analyst at JMP Securities LLC. Shares have fallen in anticipation of the expiration of the lock-ups, he said.

“It certainly wouldn’t behoove and wouldn’t be in the shareholders’ best interest to dump the shares on the market all at once,” he said. “I would assume that all of the investors that hold the 270 million-odd shares are probably rational, and probably realize that flooding the market with that kind of supply over such a short amount of time wouldn’t help their position.”

Microsoft Corp., based in Redmond, Washington, will probably hang onto its stake after the lockup-ban lifts, a person with knowledge of the matter said on Aug. 10.

Microsoft, Thiel

Microsoft views Facebook as a strategic partner in the combat against Google, rather than as a near-term moneymaker, said the person, who requested anonymity because the plans are private.

Other investors have been preparing for potential sales. Director Peter Thiel, who sold in the IPO, has given himself added flexibility to unload more holdings, according to a regulatory filing. Thiel, one of Facebook’s earliest investors, converted more than 9 million shares to Class A from Class B. Class A shares are easier to sell on the public markets.

Facebook has grappled with concerns about its valuation after reporting sales growth of 32 percent in the second quarter from the year-ago period, down from 45 percent in the first quarter and 55 percent in the fourth quarter. The second-quarter gain was dwarfed by a surge in spending on marketing and sales, which ballooned to $392 million.

Growing Optimism

Part of Facebook’s challenge is making money from the growing slice of users who access the social network over mobile devices. During the second quarter, the number of ads delivered in the U.S. dropped 2 percent from a year earlier even amid an increase in overall daily users.

Mobile advertising probably contributed just $15 million to second-quarter sales that totaled $1.18 billion, Forte said.

The company has since added a service for software developers, enabling them to entice users to download applications onto mobile devices. The company also said earlier this week that it’s testing a service that lets companies put more ads on mobile phones.

“Despite revenue growth that has decelerated, we see largely organic annual growth of” 25 percent or more through 2014, S&P Capital IQ wrote in a research note. “We have grown more optimistic about the company’s efforts related to monetization and mobile.”