i don't think i've ever actually seen a WoW player quit playing the game after they've posted a quitting post. everyone i ever played with, including myself, just stopped logging on and playing when we got bored or wanted to do other things with our time
you'll know when someone quits when they don't say anything at all and one day you'll be logged on and playing and you'll think of them, and realize that they've been gone for a while