World of Warcraft's subscriber numbers peaked around 12 million back in late 2010 and early 2011 and have been in decline since. The game slipped to 11.4 million subscribers in May 2011, then down to 10.3 million in September of 2011. While subscriber numbers continue to fall, the rate of lost subscriptions has slowed significantly.
In further clarification of the game's subscriber numbers, Blizzard President and Cofounder Mike Morhaime said that Blizzard has seen no significant change and that "December was a good month for us." This past quarter was, according to Morhaime, the "most competitive quarter ever." World of Warcraft's competition primarily came from Star Wars: The Old Republic.
Blizzard believes that the success of patch 4.3, community engagement, and the Annual Pass contributed to the subscriber retention it's seen.