I didn't say Piers was unheard of, numbnuts. Plenty of people watched America's Got Talent when he was there.
I wasted a couple minutes trying to find progressive radio ratings and didn't come up with anything. I see Rush and Hannity get 14M+. Wikipedia shows no progressive in the top 18 of radio listeners.
The difference between conservative and liberal radio hosts is substantial, since the Speaker of the House asks Rush for advice while progressive is radio is, by comparison, non-existent with the crash and burn of Air America.