The Facebook founder Mark Zuckerberg fell by $ 17.4 billion this year, while the giant means of social Media is facing a storm of attack on dealing with critics and the implications of the Russian response to the intervention of US elections.
Facebook shares fell 3 percent on Friday to $ 139.53, their lowest level since April 2017.
Zuckerberg, 34, the third richest person in the world, behind Jeff Bezos and Bill Gates, is sixth on Bloomberg's billionaire index.
Zuckerberg's wealth, which has now reached $ 55.3 billion, has fallen more than $ 31 billion from its peak on July 25.
Oracle's Larry Ellison is close to seventh with $ 54.7 billion.
Mr Zuckerberg faced fresh pressure when a group of young Democratic senators asked for news reports that the firm had used contractors to respond to critics of privacy practices and the company's efforts to thwart Russian propaganda on its network.
Against the background of these reports, the Facebook founder faces demands from investors in his network to resign as chairman and CEO.
The New York Times reported on Wednesday that Facebook had used a company called Defenders to defame critics of Facebook's policies.
Zuckerberg, for his part, denied knowing that Facebook had contracted with the company, saying that he knew this information only through the American newspaper.