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After selling 16.8 million shares for $640 million at the time of the initial public offering in May, Thiel still owned nearly 28 million shares worth about $560 million at Thursday’s trading prices.

Accel Partners invested $12.7 million in Facebook in 2005. The firm sold nearly 58 million shares for $2.2 billion as part of Facebook’s IPO and still owned nearly 144 million shares worth about $2.9 billion.

It wasn’t known how many of those shares could have been sold Thursday, and whether any of them were.

Because those investors had put up little compared with the shares’ value today, “you can understand why they would want to take some of their money off the table now,” Maher said. “But at the same time, you have to wonder if they’re thinking that Facebook isn’t much of a bargain anymore.”

Hamadeh believes the venture capitalists who invested in Facebook realize it’s a “fool’s game” to wait for a better price on the stock.

On the flip side, other key investors seem unlikely to sell additional shares right away. Microsoft, which invested $240 million in Facebook in 2007, relies on Facebook’s social network to help bring more traffic to its Bing search engine, making it less likely that it would risk antagonizing Facebook executives by bailing out. The software company also doesn’t need the money, as it is already is sitting on $63 billion in cash.

A Microsoft spokesman declined to comment Thursday.

The selling shackles will come off of an additional 1.66 billion locked-up Facebook shares during the next nine months to place more potential pressure on the stock. One of the biggest tests will come in November when about 1.2 billion insider shares will be eligible for sale.

The freed-up shares will include those owned by Zuckerberg, the Facebook CEO and founder who sold 30 million shares for $1.1 billion in the May IPO to cover his taxes.

Other Internet companies that have gone public in the past year have been hit hard by the expiration of their lock-up periods.

On Tuesday, shares of online reviews service Angie’s List suffered their biggest one-day drop so far and closed at a new low following the expiration of a similar ban. The price dropped, even though there was no word on whether any of the major investors had dumped their shares.

LinkedIn, which runs a professional networking version of Facebook, also took a big hit when restrictions on insider selling lifted last November. Its shares sank to their trading low of $55.98 after the lock-up period expired, but have since rebounded strongly. The shares closed at $103.99 Thursday, more than twice its IPO price of $45.

It appears some investors believe Facebook’s stock price is nearing its low point, at least until the next lock-up periods end this fall.

Short sellers, a type of investor that bets certain stocks will fall, had been intensifying their focus on Facebook since the company’s July 26 report of disappointing revenue growth during the second quarter. To wager against a company, short sellers must borrow the stock of other investors and then make money by paying back the shares at a lower price.

The amount of borrowed Facebook shares had spiked from about 70 million before Facebook’s second-quarter earnings report to about 92 million at the end of Thursday’s trading, said Tim Smith, senior vice president of SunGard Astecs Analytics, which tracks short-selling activity.

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