How do they measure reach and listenership?
They measure the range of the antennae based on power level and calculate the population in that area.
How do they measure listenership?
They send out little booklets to people who listen to the radio and ask them to write in the times they listen and pay them a dollar for their trouble. They then extrapolate, based on the nearby population, based on that self-selecting, filtered sampling.
If nobody listens, reach is irrelevant.