Revealed: Zuckerberg’s roommate's most expensive mistake

Facebook founder Mark Zuckerberg's college room-mate has revealed how he turned down an offer to co-found the social networking site, a decision that cost him hundreds of millions of dollars.

Joe Green, who shared a dorm with Zuckerberg at Harvard in 2004, revealed that he was asked to abandon university to start Facebook.

But Green, who still remains friends with the Facebook founder, listened to his father and stayed at Harvard while Zuckerberg moved to Silicon Valley and began his rise to becoming a billionaire, The Telegraph reports.

Green also said he had decided not to join Zuckerberg in the Facebook venture after their previous project, Facemash, got them into trouble at Harvard.

The rudimentary site was used to rate female students "hot or not."

"We'd got into a little bit of trouble with the previous project and my father, who's a professor, was not too happy with the prospect of me getting kicked out of school. Zuckerberg likes to make fun of my dad for this, but we're still very close," Green told ABC news.

Green went on to work for US Senator John Kerry's failed presidential bid, but is himself highly successful.

He now runs a multi-million dollar business called Causes, which aims to get people "civically engaged" by using Facebook to allow them to suggest charities and good causes to each other.

Meanwhile, with Facebook expected to file for an initial public offering soon Zuckerberg, 27, is now worth an estimated 17.5 billion dollars.