most function as consultants for patients with renal disease in the hospital; most places do not admit patients as part of a "renal service" (maybe in larger centers that i am not used to seeing). most oversee dialysis units and clinics (HD and PD). clinics can cover anything from pre-transplant, hypertension-related issues, and management of CRI/CRF.
most nephrologists in private practice, at least the one that takes care of 2 of my family members, do a lot of general IM management as well. One was called in for my grandmother when she needed plasmapheresis in the hospital for CIDP.
personally, i think the need for nephrologists and hepatologists will rise as this population gets "greyer"; as long as HTN and DM continue to be prevalent, those who end up needing transplants will rise. Liver disease is not as prevalent, but since most of that is chronic, the need for hepatologists down the road will be there for pre-transplant assessment, I think.
-S.