I'm not sure how many World of Warcraft subscriptions are currently active. The popularity of the game skyrocketed and then seems to have leveled off. Player numbers have certainly been bolstered by the Recruit-a-Friend promotion. I'm not convinced that it has actually brought any new players into the game. I know several people who begun additional accounts under this new scheme, and a handful of players that have returned to the game. I'm sure there are some new rookies, but they're a fraction of the new accounts.
I'm working on my third account. My friend with recruitment benefits and I have been soaring through the levels. I was planning on starting a third account come Wrath anyway, but now seems like as good of a time as any.