I quit wow because I didn't like it anymore, doesnt matter how many people there are when you only have a few thousand in your realm.

I played warhammer for the better part of a year, despite all its troubles, and right now I have an active subscription for it (but dont really play it ... lack of PvE in end game got the best of me)

I quit aion because of the grind (obviously).. no amount of subscribers is going to make me go back there anytime soon.

Right now I'm happily progressing through the ranks in Runes of Magic.. they seem to be doing alot for the games end-game.
Too bad it takes forever to get both classes to 55.

Oh and I've also got my EVE Online account.. after 5 years I've finally gotten the hang of it. I love how many people there are on the shard in eve.
I also live like 5km from CCP Iceland headquarters so I like the idea of supporting them.