Wikipedia lists over 40 B-747-400 operators and only one (NW) is American
When the A380 was launched, there were two US operators of the 747 in passenger guise - Northwest and United (Northwest was the 747-400 launch customer). However neither had taken a new one for years nor had any more on order - they were already looking at the transition to big twins.
Both United and Delta (which merged with NW several years back) have completely phased out the 747 by the end of 2017. It's rather telling that not only were they replaced by big twins - in many cases they weren't even particularly big 'big twins' - many of those routes now being flown by 767, 777-200, and A330s.