Peter Thiel, one of Facebook's earliest investors and a member of its board, was among the insiders selling stock in the social network after a lockup expired last week.

A regulatory filing on Monday said that Thiel, a former CEO at PayPal sold about 20 million shares of Facebook through affiliates such as his Founders Fund and other entities last week. The shares were sold on Thursday and Friday for between $19.27 and $20.69, for a total of about $396 million.

Though Facebook's stock has lost nearly half of its value since its initial public offering, Thiel made out well. As one of Facebook's initial investors, he first invested $500,000 in the company in 2004. He sold 16.8 million shares in the company's May initial public offering for about $640 million. With last week's sale, Thiel has now sold most of his stake in the company.

Thursday marked the expiration of the first of several lockup periods barring insiders from selling Facebook's stock following its IPO. All told, investors who owned a combined 271 million Facebook shares became eligible to sell their stock.

Accel Partners, another early investor, said in filings that it distributed stock to its limited partners after the lockup expired. It was not clear from the filings what the partners then did with the shares, though it's likely that some sold based on Facebook's more than 6 percent stock price drop and high trading volume Thursday.

On Monday, Facebook's shares hit a new low of $18.75 before bouncing back to $20.01 by the time the stock market closed. That's up 96 cents, or 5 percent, for the day.

The social networking icon's much-anticipated IPO turned sour amid technical problems on the Nasdaq stock market and high expectations. The stock has not topped its $38 IPO price since its first trading day.

Investors are concerned about Facebook's ability to grow revenue quickly, especially through mobile advertising as users flock to smartphones and tablet computers. There are also looming expirations of the IPO lock-up periods. Lockups preventing many Facebook employees from selling stock expire this fall, potentially flooding the market with more shares.

Another 1.66 billion locked-up Facebook shares will be freed up for sale over the next nine months, likely to place more 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.

Thiel hasn't been keeping his proceeds from Facebook's public offering to himself. According to Politico, Thiel donated $1 million to Club for Growth Action, a super PAC aligned with the Tea Party, in July. Thiel was also the "single largest donor" to Endorse Liberty, a super PAC created to support Ron Paul's presidential bid, Forbes reported earlier this year. Thiel donated $2.7 million to the PAC. His fellow PayPal co-founders, Luke Nosek and Scott Banister, also donated to Endorse Liberty. (AP)