Activision-Blizzard made the press release
for their quarterly results (Q4 2012) available before the actual conference call. On page three, you will find the announcement about the WoW subscribers, which they define as follows (on page five):
World of Warcraft subscribers include individuals who have paid a subscription fee or have an active prepaid card to play World of Warcraft, as well as those who have purchased the game and are within their free month of access. Internet Game Room players who have accessed the game over the last thirty days are also counted as subscribers. The above definition excludes all players under free promotional subscriptions, expired or cancelled subscriptions, and expired prepaid cards. Subscribers in licensees' territories are defined along the same rules.
I'm a bit disappointed by the number, I have to say. I find Mists of Pandaria to be a really good expansion and things will only get better with Patch 5.2 (the new daily quest hub is just awesome, so far).
One thing I regret is that they never release the numbers by zone. In particular, I would like to know how the number of subscribers in Europe and in North America has varied over the years (mostly because this is the readership we target with this website).