Facebook shares rose nearly 13 percent on Wednesday when insiders failed to flood the market with stock as expected and investors who sold the company short had to make good on their bets.

Facebook shares were up 12.9 percent to $22.36 at the close of the Nasdaq trading day and held the gain in after-market exchanges.

It was the second-best session for Facebook since its stock market debut in May, second only to its jump of nearly 20 percent in October after posting earning results that topped expectations.

Some analysts said Facebook shares climbed Wednesday on the backs of investors who sold stock short, anticipating prices would fall at the end of a lock-in period that had prevented the sales of more than 800 million shares held by social network employees.

"People were scared about the lock-up; a lot of people shorted (the stock)," said Social Internet Fund analyst Lou Kerner.

"As the lock expires, the shorts need to cover, and they are having to pay up to cover because the sellers aren't swamping the market."

In an effort to avoid big swings in its stock price, Facebook's initial public offering came with a set of lock-in periods during which shares held by investors or employees could not be sold.

One of those periods expired on Wednesday, when 804 million shares held by employees became eligible to be traded.

Facebook stock price dropped when two prior lock-in periods ended in August and October, so some investors essentially sold borrowed shares expecting to buy them back for less on Wednesday.

"The bottom line is there is no sure thing, and the shorts that held on got it wrong this time," Kerner said.

When sellers didn't flood the market as expected, investors "covering" short-sales of Facebook stock created buying pressure that pushed the price higher, according to analysts.

Some analysts saw Facebook shares as a promising long-term investment at well below the $38 price at which they made their stock market debut.

The California-based company has shown signs that it is dealing with what was considered a troubling flaw, figuring out how to make money off users increasingly using smartphones or tablet computers to connect with the leading social network.