It blows my mind that the word "paltry" would be applied to WoW's subscriber base. It dwarfs even its biggest competitors by an order of magnitude!
However if you consider the larger online gaming market I would consider WoW's main competitors the likes of Zynga. Farmville alone has 28.4M monthly active users.
http://blog.raptr.com/2011/09/27/zynga-report/
Specifically, ~20% of WOW players also played a Zynga game, and ~30% of XBox players also played a Zynga game.