By Matt Jarzemsky and Drew Fitzgerald 
 

Facebook Inc. (FB) shares jumped as much as 11% Wednesday as about 804 million shares held by early investors became eligible for trading.

The sharp gain in stock price goes against conventional thinking, which suggests an increase in the supply of shares should ease demand and ultimately reduce the share price. Of course, few things regarding Facebook's short six-month history as a public company has gone as expected or without confusion.

Among the reasons cited for the stock increase Wednesday were pent-up demand for Facebook stock and a "squeeze" on those betting that Facebook shares would fall following the lock-up expiration.

"If you have people betting on a big downturn, and it doesn't materialize, the next move is up," said Steve Sosnick, equity risk manager at Timber Hill, the market-making unit of Interactive Brokers Group Inc. He added that investors unwinding bearish bets on the stock might be contributing to the rally.

Short sellers borrow shares and sell them, hoping to buy lower-priced shares after they decline in price and return those to the lender, pocketing the difference. But short sellers are on the hook for losses if the stock rises instead. Certain options bets also can lose speculators money if a stock rises.

"It's such a day-trader type of name, you get a little rally and it jumps off," Mr. Sosnick added.

Facebook shares recently rose 9.8% to $21.81. The stock hit a high of $22.09, its first move above $22 this month. Of course, shares remain well-below the stock's initial price of $38 on May 18.

Facebook shares saw heavy trading volume, although the pace had slowed significantly from the session's first half-hour. As of early Wednesday afternoon, 148 million shares had traded hands, which already makes it the fifth busiest trading day for Facebook stock.

The volume Wednesday is nearly three times Facebook's average over the past 30 days, according to FactSet. By comparison, Bank of America Corp. (BAC), the most heavily traded stock in the Standard & Poor's 500-stock index, had seen just 111.8 million trades.

Pent-up demand for the stock was cited as a possible catalyst Wednesday. "There was a lot of money that was waiting on the sidelines until today," Pivotal Research's Brian Wieser said. Some investors may have been waiting to buy Facebook stock, expecting cheaper prices Wednesday because certain early investors would finally be able to sell their stock.

A shift in investor sentiment regarding Facebook may be adding support, too. "In the past, we didn't have the fundamental support" the stock enjoyed in recent weeks, Sterne, Agee & Leach analyst Arvind Bhatia said.

Following a five-month slide, Facebook shares have held up better over the past month after the company's third-quarter earnings showed its business had become more profitable on mobile devices, a sign Mr. Bhatia said shows management is "on top" of its users' growing use of smartphones.

Also possibly contributing to the gains Wednesday was a market rumor, according to traders, that there was a delay in delivering some of the 804 million shares to investors, potentially tightening supply. A Facebook spokeswoman declined to comment, saying the company doesn't respond to market rumors.

Options traders, meanwhile, have been betting that the worst has passed in terms of pressure on the stock from lockup expirations, or the lifting of rules that bar early investors from selling certain shares into the market.

"The lockups have been talked about so much, for months and months," said Brian Overby, senior options analyst at online brokerage TradeKing. "Just because the stock is being unlocked doesn't mean everyone is going to sell."

Those who bet the lockup expiration wouldn't be so bad for the stock are looking pretty good. Monday's actively traded November $20 and $21 call options, which were bought at an average cost of 30 cents and 17 cents/share, respectively, could now be sold for about $1.75 and 90 cents, a profit of more than 400%.

--Kaitlyn Kiernan contributed to this report.

Write to Matt Jarzemsky at matthew.jarzemsky@dowjones.com or Drew FitzGerald at andrew.fitzgerald@dowjones.com

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