Mark Poker's success, Mark Zuckerberg, fell this year to $ 17.4 billion, while the social media giant faces a storm of attacks on visitor care and the consequences of its reaction to Russian intervention in the US election.
Facebook shares fell 3% on Friday to $ 139.53, the lowest level since April 2017.
Zuckerberg, 34, the third richest man in the world, is behind Jeff Bezos and Bill Gates, the sixth on the Bloomberg billionaire index.
Zuckerberg's wealth, which now stands at $ 55.3 billion, fell more than $ 31 billion from its peak on July 25.
Oracle's Oracle Oracle is close to the seventh with $ 54.7 billion.
Mr. Zuckerberg came under new pressure when a group of young Democratic senators asked for news reports that the ministry used contractors to respond to critics about privacy practices and the company's efforts to thwart Russian propaganda in its network.
Against the background of the reports, Facebook founder is facing demands from investors in his network to resign as chairman and CEO.
The New York Times reported on Wednesday that Facebook has used a company called Defenders to denigrate visitors of Facebook's policies.
Zuckerberg, for his part, denied that he knew that Facebook had contacted the company, saying that he knew only through the American newspaper.