I am not good with figures but what I can see on the inet is there are about 10 million ereaders worldwide. Half of them in the US. Even if it were all Kindles and nooks, makes the rest of the readers epub devices.
Over here (Europe) it is the standard. And no extra B&N DRM is known simply because here is no B&N. Sony doesn't even bother to make all their devices with wifi because the rest of the world is not wifi crazy as the Americans are. See interview
here.