What do you do once a bar of soap becomes really small?

Then the remainder I try to "meld" with a new bar.

If that doesn't work, I throw the remainder out. I wouldn't put it down the drain, because we've had a lot of problems with clogged drains over the years, sometimes to the point of it being a nightmare for a day or two, where we haven't been able to use the shower (we only have one at home) as the landlord's flunkies try to fix it, etc. I wouldn't want to do anything that would promote hair coagulating more than it already does.