I never really thought about this until I read this thread.

I sometimes only use the foam earplugs or toothbrush/toothpaste in those business class toiletry kits. After I'm done with them, I put the earplugs and toothbrush back in the case and just leave the kit on the seat or somewhere. I just assumed that airlines tossed away all of the kits after the plane landed? Or do they pick up the "unused" ones and reuse them for the next flight?

I only fly business class on TATL and TPAC flights.(usually United, ANA, Lufthansa and BA)