I noticed this when I used to play Everquest Online Adventures (EQOA), but now this is occurring in WOW.
You meet someone in game, hang out, quest, visit instances, and just in general have a good time. Then one day they vanish. They stay dark on your friends list, never answer your mail, and basically never log in again.
Did they quit WOW?
Did they get hit by a falling meteor from the sky?
Did they get whacked with the Blizzard ban stick?
But because you only knew their WOW character, you have no way of knowing.
This is what happens in the virtual world. I guess this is just the way it is, but I have to admit this bugs me.