More importantly, 11.5 million people play WoW. That's a MASSIVE player base and, given that the type of game is a massively multiplayer online game, that "massive" part is kind of important.
And how many of those players play on one same shard? The massive part matters only if I can actually interact with those other players and the biggest US realms have about 35k characters of levels 10+ rolled on it. That's characters only, mind you.
I don't have any numbers on it, but if you could count only actual accounts/players (and/or players logged in at the same time), those numbers would be way less than that, too.
Anyway, if you're gonna go with this argument, EvE Online beats the whole bunch in that department without breaking a sweat. It's where the massively multiplayer aspect is truly massive with a peak of around 45k accounts (probably a bit less if you discard alt accounts) logged in at the same time in one persistent game universe.