A couple of points:
1. Currently, only black holes small enough to have a Hawking Temperature greater than the (black-body) temperature of the Cosmic Microwave Background (CMB) could possibly be reducing their mass (all those bigger will actually be accreting Energy/mass from the CMB and, presumably, the background neutrinos). The position of this divide will change as the CMB ‘cools’ with the continued expansion of the Universe.
2. We really do not quantitatively know what will happen to very small black holes, since we do not have a complete or consistent quantum theory of gravity. Hawking Radiation is a prediction using an approximate analysis using quantum theory and assuming only a slight curvature (perturbation) of ‘flat’ space in the vicinity of a black hole event horizon. Hence, at best, it is only approximately valid for large black holes, and almost certainly invalid for small ones where the curvature is severe…..