As everyone knows WoWs pre85 stuff is pretty useless, so all you see is alts leveling and it's about 20 or so people per zone. Then you have a thousand or so people in the cities, 100 or so in every cata zone, thousands in BGs, thousands in random dungeons. And that is per server, there are probably 50+ servers NA alone. Also PVP is linked across every single server, and Dungeon Finder is linked across 2-4 battlegroups which = 20-40 servers. Pretty much every other MMO you see 100 people in the most crowded area, 20-30 in other crowded areas, and 1-2 in every other zone, and usually there is only a handful of servers.