The other complication is that when Blizzard releases sub numbers, they don't break that down for the public by the age of the sub (i.e.: First month only, 2-6 months, 7-12 months, >12 months). Without that breakdown, we really can't tell which segment of people are leaving.

My suspicion is that it's the long-term casuals that are leaving. New subs continue to inflate numbers as it is still a popular game PR-wise.