? Billionaire Peter Thiel was Facebook’s first outside investor, but he may now have regrets for how soon he cashed out. Just months after the tech company’s IPO, he sold millions of shares for $20 each for a sum of $400 million; with the stock now worth 37 times more, he could have been $15 billion richer today.

Investing is a game of risk and reward. Get it right, and you could build a billion-dollar fortune. Get it wrong, and it might all go up in smoke.

For PayPal cofounder Peter Thiel, being the first outside Facebook investor was hardly a bust—but there may be some seller’s remorse.

The tech entrepreneur first coughed up $500,000 in 2004 for a 10% stake in the company, just months into its creation—giving it a value of about $4.9 million. Nearly a decade later, following the company’s May 2012 IPO, Thiel decided it was time to cash out. Thiel sold about 20.1 million shares in the company—a majority of his stake—for $19.27 to $20.69 each, netting him about $400 million at the time.

Had he held his shares until today, they would be worth about $14.76 billion (Meta’s stock price is $736 at the time of writing), a decision that may haunt even one of Silicon Valley’s savviest investors.

What led him to invest in the first place? He believed the college market—which was Facebook’s original audience—had been underestimated.

“I think investors always have a bias to invest in things they themselves use and they undervalue things they don’t use so there aren’t many investors who are in college,” he said to This Week in Startups in 2015.

Fortune reached out to Thiel for comment.

Facebook is a billionaire farm

Despite selling a majority of his stake in the company, the 57-year-old remained heavily involved for many years, serving on the board until 2022.

“Peter has been a valuable member of our board and I’m deeply grateful for everything he has done for our company—from believing in us when few others would, to teaching me so many lessons about business, economics, and the world,” Facebook cofounder and CEO Mark Zuckerberg said at the time of Thiel’s stepping down.

Today, Thiel’s net worth sits at about $21.7 billion, according to Bloomberg, thanks in part to the billions he made from being the cofounder of PayPal and Palantir.

And he’s not the only investor to have thrived off the success of Facebook; the social network’s growth has created countless millionaires—and multiple billionaires.

Of course, no one has benefited more financially than Zuckerberg, who is currently the second-richest person in the world, according to Bloomberg, with an estimated net worth of $260 billion. In fact, his worth has increased more than any other billionaire this year, with a year-to-date growth of $52.8 billion.

But the company’s lesser-known cofounders have also joined the billionaire club, including Dustin Moskovitz ($11.4 billion) and Eduardo Saverin ($40 billion), as well as former COO Sheryl Sandberg ($2.5 billion) and former president Sean Parker ($3 billion).

The wrong sale at the wrong time

While it’s possible Thiel regrets selling in 2012, nothing might dwarf the early pullout from Apple’s third cofounder, Ronald Wayne.

He cashed out his 10% stake in the computer company for $800 just 12 days after signing the contract. Wayne’s share could now be worth between $75 billion and $300 billion, thanks to the company’s current $3 trillion market cap (his shares would have likely been diluted thanks to new investors and public offering).

And while Wayne has said he had no regrets at the time since he figured he would one day just be the “richest man in the cemetery,” he’s since admitted it would’ve been nice not to worry about money. To make ends meet, he’s relied on renting out part of his property, as well as cashing his monthly Social Security check.

“I’ve never been rich, but I’ve never been hungry either,” he told Business Insider.