Net Worth of 400 Richest Americans Jumped 13% And those 400 now make up 1/8 of the US economy By Kevin Spak, Newser Staff Posted Sep 19, 2012 11:55 AM CDT 49 comments Comments Bill Gates and Warren Buffett, seen in this file photo, are the top two names on the Forbes 400. (AP Photo/Paul Sakuma) (Newser) – It was a good year for America's 400 richest people, who saw their net worth jump 13% to $1.7 trillion, according to Forbes' latest "Forbes 400" list. For a little context, Reuters notes that that means their combined net worth represents roughly one-eighth of the entire US economy. The top five names on the list didn't change this year—it's still Bill Gates, Warren Buffett, Larry Ellison, and the Koch brothers, in that order. Perhaps the most notable change: Mark Zuckerberg plummeted from No. 14 to No. 36, thanks to Facebook's less-than-stellar NASDAQ debut.