France went into the game already sure of top spot but outscored the Americans by five tries to two to make sure of a fourth bonus point in four matches.
Liebenberg, a controversial selection for the World Cup ahead of Thomas Castaignede, scored from close range each time after sweeping back moves.
The Eagles, however, 26-0 down at the break, fought back with two tries early in the second half, flyhalf Mike Hercus setting up the first with a pop pass before going over himself after a neat dummy inside.
It was the first time in the tournament the French failed to pass 50 points.—Reuters