Facebook CEO Mark Zuckerberg took just one year to turn from a millionaire to a billionaire, according to a study by Betway based on Forbes' data. Zuckerberg was the youngest self-made billionaire on Forbes Billionaires list when he debuted in 2008 at the age of 23. Zuckerberg is currently world's 7th richest person with a fortune of $63.8 billion.