It was originally from me and Cinnar laughing at how creepy it is when someone logs out. They just went POOF! we would say. Then after someone would leave we said, "Poof! They're gone."
And so became my thing. Like Ari says "Oy" I say "Poof". It really is funny that I influenced so many people, and some, don't even know why! And that is the story of "POOF."