Mark Zuckerberg’s personal wealth has fallen by more than $6 billion in a few hours, knocking him down a notch on the list of the world’s richest people, after a whistleblower came forward and outages took Facebook Inc.’s flagship products offline.

A selloff sent the social-media giant’s stock plummeting 4.9% on Monday, adding to a drop of about 15% since mid-September.

The stock slide on Monday sent Zuckerberg’s worth down to $121.6 billion, dropping him below Bill Gates to No. 5 on the Bloomberg Billionaires Index. He’s down from almost $140 billion in a matter of weeks, according to the index.