One day in 2005, a young man named Mark Zuckerberg surprised the world with a new concept called social media. He called it Facebook, which has grown to have an estimated market value of $100 billion now.

The 21-year-old Zuckerberg, who had just established the Facebook after dropping out from Harvard, met the 60-year-old Donald Graham, CEO of Washington Post, in the latter's office in 2005. That is how the Facebook love story started, according to a report by The Wall Street Journal.

Though the age gap between Zuckerberg and Graham is 39 years, they have been mentors for each other for the past seven years. As Facebook expanded in 2007, Zuckerberg, who had no business experience, sent an e-mail to Graham, saying I'm a CEO now and would like to shadow you and see what you do.

Graham invited Zuckerberg to his office, let him attend executive meetings, an investor conference, walk the newsroom, and look around in the print man's book-lined office on the ninth floor of the Post building. And at the end of 2008, Zuckerberg asked Graham to join the board of Facebook, saying that he admired the management philosophy of Washington Post Co. of making society more open and understanding and not just business.

Zuckerberg did not forget his debt to Graham, who helped him to understand the life of a CEO. When the Post Co. became cash-strapped, Zuckerberg shared with Graham new business ideas, especially the power of social media and news-sharing apps.

Though Graham also shares a close relationship with Warren Buffett and often turns to him for advice, he says Mark is an adviser in different ways. Indeed, Zuckerberg has helped Graham open his eyes to the digital world.

Facebook, which used to get management advice from Graham, has now grown up to be giant, having a maket value of $100 billion, dwarfing the Washington Post, whose market value is $2.9 billion. But the CEO of two companies still continue to have a connection like father and son, always mentoring and helping each other out.