The mass doesn't disappear,...
Brings up a question.
Does a black hole ever 'release' the matter and energy it 'absorbs' ?
Yes, as PreciousLiberty mentioned Hawkings radiation will slowly (very slowly) evaporate a black whole. He did throw in that this evaporation is currently over whelmed by the cosmic background radiation, which I was unaware of (and I am thinking about). Also current theory is knowledge is not lost in a black whole either, Hawkins was proven wrong there.