Joe Green was college roommates with Facebook founder and Wall Street golden boy Mark Zuckerberg.
Like that scene in the movie, The Social Network, Joe was there when Zuckerberg came up with the idea that would change the world.
However, Joe declined Zuckerberg's offer to drop out of Harvard to help run Facebook.
With the company filing to go public Wednesday, and investors drooling over the chance of buying Facebook shares, Joe's decision has cost him an estimated $400 million. About the loss, Green admitted that, "it would be inhuman not to have moments of regret."
Zuckerberg's fortune - already put at $17 billion - just grew by many billions more.
Financial expert Nicole Lapin told INSIDE EDITION, "this is one of the greatest companies ever created, likely others will follow suit, but this is the big one."
Joe's story is similar to that of Ron Wayne, who founded Apple computers with Steve Jobs and Steve Wozniak in 1976, but bailed out before the company hit it big.
His decision cost him an astounding $35 billion.
Wayne now lives in a modest home outside of Las Vegas. "I live on my Social Security," he says.
Joe Green, meanwhile, is back with his old roommate Zuckerberg, running a business that's partnered with Facebook.
He may be out $400 million but he owns shares in the company, and like many Facebook employees, he stands to do very nicely when Facebook goes public. Green has said, "I have enough shares to be very comfortable."