According to a filing with the SEC on Tuesday, Thiel sold roughly three-quarters of his remaining Facebook stock on Nov. 20, netting him $28.7 million.
The PayPal co-founder and venture investor was an early backer of Facebook, investing $500,000 in 2004, gaining a board seat that he still holds and 2.5 percent of the company at the time of the IPO.
While Thiel is in no way hurting for cash, he would have been much wealthier had he stashed those Facebook shares away after the 2012 IPO.
Instead, as part of a pre-arranged plan, he sold almost 80 percent of his stake within a few months of the IPO at an average price of less than $20 a share. Facebook is now trading at $180.50.
Looking back five years, his timing was terrible. Facebook shares fell sharply soon after the IPO, as investors doubted the company's ability to transition its advertising business to mobile. They were very wrong.
The shares Thiel sold for close to $400 million would now be worth over $3.6 billion.
"He and his firm missed out on a lot of upside," says Ben Silverman, director of research at InsiderScore.com.
Forbes estimates Thiel's worth at $2.6 billion, thanks largely to his non-Facebook activities. Thiel put his money to work as a hedge fund manager and venture capitalist. His venture firm, Founders Fund, was an early investor in Palantir, one of the most valuable technology start-ups.
An email to Thiel seeking comment wasn't immediately returned.
Thiel held 220,718 Class A Facebook shares and 54,995 Class B shares as of March 31, according to the company's annual proxy filing. He sold 60,300 Class A shares in July.
-- CNBC's Anita Balakrishnan and John Melloy contributed to this report.