"He is the youngest billionaire in the world right now and we also believe he is the youngest self-made billionaire in history," said the magazine's Associate Editor Matthew Miller, unveiling this year's super-rich list.
The magazine put the former Harvard student's personal wealth at 1.5 billion dollars, based on what it said was a conservative valuation of five billion dollars for Facebook and Zuckerberg's estimated 30 percent stake.
It played down speculation that the site could be worth as much as 15 billion dollars, which was based on Microsoft paying 240 million dollars for a 1.6 percent stake in the company last year.
"Would it really fetch that much today? Some analysts — and a few Facebook investors — doubt it," the magazine said. It said it based its valuation on Facebook's estimated annual sales of 150 million dollars.