"While the lockup is expiring, there is nothing requiring anybody to sell," said Tim Ghriskey, chief investment officer at Solaris Group in Bedford Hills, New York. "Given the low price, these long-term holders are deciding to hold the stock, and that is lifting it here as the fear of the expiration subsides."

Roughly 800 million Facebook shares were eligible for sale on Wednesday after restrictions on insider selling were lifted on the biggest block since Facebook's May initial public offering.

The lockup expiration greatly expands the 921-million-share "float" available for trading on the market until now.

"We've seen this before with other lockups. People sell them leading up to the lockup period expiring, and then they have a bit of a relief rally," said Ryan Jacob, chief executive of the Jacob Funds, which does not own Facebook shares.

In August, shares of the online reviews website Yelp Inc surged by more than 20 percent on the day that insider trading restrictions expired. That stock's rally was boosted as short-sellers scrambled to cover their positions when the expected flood of selling failed to materialize, say analysts.

Facebook shares finished Wednesday's regular trading session up 12.6 percent at $22.36 on the Nasdaq, with trading volume for the stock more than four times the average during the past 50 days.

The world's No. 1 online social network became the only U.S. company to debut with a market value of more than $100 billion. But its value has dropped nearly 50 percent since the IPO on concerns about money-making prospects over the long term.

Insider trading lockup provisions started to expire in August, and the rolling expirations have added to the pressure on the stock.

Restrictions on insider selling have expired in waves. A limitation on more than 200 million shares expired on October 29.

COST OF SHORTING

Pivotal Research Group analyst Brian Wieser said he did not expect Facebook insiders to sell all of their shares as the lockups expired.

"I would expect heavy volumes over the next few weeks, but not undigestible volumes," said Wieser. By his estimate, roughly 486 million of the nearly 800 million newly freed Facebook shares will be sold.

There is some evidence the heavy interest in "shorting" the stock was dissipating, given the poor performance since it first sold shares in May. Investors who believe a stock will fall can bet against it by shorting the stock - that is, borrowing it and selling it in the hopes it will decline.

According to Markit, a financial information services company, about 28 percent of the shares available for short-selling were being borrowed for that purpose, down from a high of more than 80 percent in early August.

Similarly, SunGard's Astec Analytics, which also tracks interest in shorting, noted that the cost of borrowing Facebook shares is down more than 50 percent since the beginning of the month.

"Everything would seem to indicate the market is losing its appetite to short Facebook," wrote Karl Loomes, market analyst at Astec.

The cost of shorting Facebook has declined to 0.18 percent on an annualized basis, Astec said on Wednesday. By contrast, shortly after the IPO, the cost to short the stock ranged from 40 percent to 50 percent annually.

"It's become somewhat of a controversial stock. It always adds fuel to the fire if you have a sizable short position," said Stephen Massocca, managing director at Wedbush Morgan in San Francisco.

"But the unlock is not new news. It doesn't mean everyone is going to sell, and it doesn't mean every order is going to come in today."

Facebook, with roughly 1 billion users, has faced a tough reception on Wall Street amid concerns about its slowing revenue growth and nascent advertising efforts on mobile devices.

But the company delivered better-than-expected third-quarter results on October 23 and revealed that 14 percent of its advertising revenue is now from mobile ads, reassuring some investors it was beginning to figure out how to earn money from smartphone and tablet users.

"They had a pretty good quarter. I think a lot of investors, though, are waiting to see some consistency in the results," said Jacob.

Several members of Facebook's senior management have sold millions of dollars' worth of shares in recent weeks through pre-arranged stock trading plans as lockup restrictions expired.

Chief Operating Officer Sheryl Sandberg has sold roughly 530,000 shares this month, netting just over $11 million, though she still owns roughly 20 million vested shares in Facebook.

In August, Facebook board member Peter Thiel sold roughly $400 million worth of Facebook stock, the majority of his stake, when an earlier phase of lockup restrictions expired.

Facebook's 28-year-old chief executive, Mark Zuckerberg, has pledged not to sell any shares before September 2013.

"I'm sure we're seeing some selling from guys whose shares are unlocking, but the supply is not nearly as much as everybody expected," said Arvind Bhatia, an analyst with Sterne, Agee & Leach.

(Editing by Jeffrey Benkoe, David Gregorio and Matthew Lewis)

• A Permanent Economic Emergency