Come to think of it, that 2 million in North American actually feels low to me. If you add up the populations of the US and Canada and divide by 2 million, you get 167: one in every 167 US/Canada residents is a WoW player, approximately. It feels like more than that to me, but that probably says much more about the kind of people I know than anything else.
I can't help but think back to when I first heard about WoW, when Blizzard announced it as their next project. I was upset, since I figured I would never want to play a game that required a subscription fee (I was a huge Diablo II fan at the time), and like everyone else, I underestimated the size of the MMO market in general. Which just goes to show, when Blizzard announces that their next project is a cross between NiGHTS Into Dreams, Cooking Mama and DDR, don't doubt them. They'll find a way to pull it off.