Facebook CEO’s worth down to $120.9 billion, dropping him below Bill Gates to No. 5 on the Bloomberg Billionaires Index. Mark Zuckerberg’s personal wealth has fallen by nearly $7 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 around 5% on Monday, adding to a drop of about 15% since mid-September. The stock slide on Monday sent Zuckerberg’s worth down to $120.9 billion, dropping him below Bill Gates to No. 5 on the Bloomberg Billionaires Index. He’s lost about $19 billion of wealth since Sept. 13, when he was worth nearly $140 billion, according to the index. On Sept. 13, the Wall Street Journal began publishing a series of stories based on a cache of internal documents, revealing that Facebook knew about a wide range of problems with its products — such as Instagram’s...