I'd imagine a lot of WoW's current userbase are folks who first started playing 2-3 years ago, still have friends who play, etc; the game is relatively new to them in many ways and the social aspect remains strong. It has lost its luster for people who started playing long ago such as myself. Most of my WoW friends moved on to LoL and other f2p games. Interestingly, I don't know anyone except Etoilet who moved to GW2.
There used to be a general assumption that "WoW killers" were always quite flawed, or if a WoW got involved in one the assumption was "he'll be back in two months." Conan, Warhammer, Aeon, Rift, etc all followed that pattern. But years of repetitive gameplay and a stale world has really taken the shine off WoW, to a point where I rarely come across any diehard fans anymore. You can pretty much do all the things you did in WoW for free in other games now, so why pay monthly just to stand around in a capital city?