According to Forbes, the richest 400 Americans’ net worth increased 13 percent in 2012 to $1.7 trillion. The average net worth of the group was also up to a record $4.2 billion, and collectively they make up the equivalent of one-eighth of the U.S. economy. So who makes the cut? Bill Gates tops the list for the 19th year in a row, with a net worth of $66 billion, followed by Warren Buffett ($46 billion), Larry Ellison ($41 billion), and the Koch brothers tied for fourth at $31 billion. Bucking the trend of increases is Facebook CEO Mark Zuckerberg, whose net worth fell by half to just $9.4 billion and slid to No. 36 on the list from No. 14 in 2011.