Maybe it's more like phone numbers - they just get recycled. But since the combination possibilities are so vast, nobody would actually notice when one gets recycled.
Anyhow, I have nothing to support this, and it is just a quick guess. Really, I'm trying to get this question back near the top, as I'd love to see what others have to say. I've never seen this question before, and am interested in other responses. Hopefully someone else will even possibly have some 'facts' to back up their answers. Sorry I couldn't help in THAT regard.
