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Paul, this may come as a shock but they go to sock heaven, just as all the missing down the back of the sofa, items such as ball point pens, combs etc. they all having performed their duty to mankind* go to their own version of heaven. Kryton from red dwarf explains this phenomenom in very simple terms if ever you get the chance to watch the re-runs.

* Gayle please note it is still referred to as MANkind you as all other simpler organisms and devices will get your reward in a similar version of heaven!

(Oh, boy, I'm going to get an ear full for that little bit).