In the countless discussions about subscription numbers that have been posted on MMO, here is all the information in a few sentences -
1) TBC had close to 12 million subscription numbers
2) WotLK didn't get released in China
3) China made up around 6 million subscribers, meaning WotLK immediate subscription was around 6 million
4) VGChartz reported 6.42 million retail copies of TBC sold, whereas WotLK has only sold 5.24 million. However this is limited to retail sales only, and official records stopped on the 12th July 2009.
5) Blizzard have never released ANY information on number of subscribers after the "11 million" figure that was released at the end of TBC.
Unless someone, somewhere has any new information to add, this is all the information we are likely to ever know.
edit - And yes,
http://razetheworld.gameriot.com/blo...List-Sept-2009, goes to show that Aion isnt the joke that some hardcore WoW players like to pretend it is. Unofficial rumours suggest Aion's playerbase is somewhere around 4 million, which could be remarkably close to what WotLK currently has.