And the behemoth finally drops below 10 million, coming in at 9.1 million active subscribers. I know a lot of people are just going to jump on the "All MMO's are dying" train but there's more to this than just MMO's aren't popular anymore.
Cataclysm was a bad expansion. First of all, the most basic content was very difficult in the beginning which lead to a ton of people quitting. But more importantly there's not a lot to do. Cataclysm consistently had a lack of content. They focused too much on revamping the 1-60 quest zones and didn't put a lot of work into new things to do in the end game. If you didn't want to make a new character there just wasn't much to do.
It's gotten much worse in the past few months though. There hasn't been any new content since late November when patch 4.3 released. All of the hardcore raiders are done the the latest raid instances and PVP'ers have every piece of gear they can get. This dead period with nothing to do has been going on for a really, really long time now. Without any content there's no reason to keep playing.
I suspect MoP will bring back a ton of subs though. The new expansion is the largest ever, it's amazing how much new stuff there is to do. You have the basic stuff, 16 new raid bosses which is what most people care about, as well as challenge mode dungeons, new heroics, a full on Harvest Moon side game, a full on Pokemon side game, a new race and class, a bunch more stuff that will keep people around better than Cataclysm did, which really only had Raids.
EDIT: I just looked at a chart showing subscription rates. WoW has been slowly declining since the start of 2011, which is soon after Cataclysm Launched. Q2 of 2012 has a this massive 1.1 million drop of, further showing that this dead period really is what lead to so many people cancelling subs.