Another feature of small black holes is that they are unstable. This was explained in some detail by Bill Unruh in the UBC Physics Department. The basic idea is that for a small black hole the tidal forces at the Schwarzschild radius are so enormous that they can tear apart the vacuum - that is, pull one of the partners in a "virtual pair" or "bubble" down into the black hole while the other escapes as radiation. The resultant energy loss is deducted from the mass of the black hole, making it still smaller. This is a runaway process that ends in a rather impressive explosion. Not to worry, all the small "primordial" black holes (made in the Big Bang) have by now decayed. On the other hand, a marginally larger primordial black hole might have taken until now to get down to a size where the radiation really starts taking off....