There are high pop servers on US. Just did a quick Census check on Dalaran-US Alliance... 2380 online... http://imgur.com/7KhlFoW
Horde side... http://imgur.com/NWNvo6z (Horde love their Blood Elves)
3806 server wide, and of course not counting the people leveling below level 20. So more like 4000-4100. Not too bad.