Peter Thiel, one of Facebook's earliest and most important investors (as well as a board member), has unloaded most of his stock in the company, which is leading some analysts to worry that he may be giving up on the social network. A recently released SEC filing shows that he sold more than 20 million shares last week, which combined with the shares he unloaded during the IPO have given him about $1 billion in cash from his initial $500,000 investment. But it's reduced his stake in the company to a small fraction of what it once was.

You may remember Thiel's cameo appearance in The Social Network (as portrayed by actor Wallace Langham) in the most dramatic angel investing scene in cinematic history. Thiel, a PayPal founder, gave the fledgling company its first big financial boost back in 2004 and stayed with them as an important mentor and guide to an inexperienced management team. He isn't just an investor, he's also board member — which is why his decision to divest so much of his holdings has some people wondering about his long-term commitment to Facebook.

The timing also doesn't look great given the current predicament of Facebook's stock. Thiel made just under $400 million on his most recent stock sale, but that's less than half of what he might have made had he sold them all after the IPO. (The stock hit a new low on Monday, before bouncing back above $20 a share. It opened at $38 in May.) However, Thiel reportedly planned this major sale back before the stock went public, so he isn't bailing just because of the lousy stock performance. He also made his initial investment eight years ago, which is a long time for a venture capitalist to hold on to something without being able to realize the gains. 

While no one is questioning Thiel's right to take back what he earned, some people are still wondering why a board member would choose to essentially exit the company (financially, at least) so soon after going public. And before regular employees will get the same option. Their shares are "locked up" until at least at October. Thiel still owns a not-inconsiderable 5.6 million shares, but his stake in the company is significantly reduced. It feels to some like a vote of no confidence from someone who played such a key role in the building of the company and now looks ready to move on to other things.

Even if he is planning to move on, the move may not signal anything about Thiel's opinion of the future (maybe he's just bored?), but given all the pessimism surrounding Facebook at the moment, it's just one more piece of downer news that they don't need.