or even for tv for that matter.

AM/FM/Shortwave (can any ratings system cover SW?) or even HD...some
say that any new technology will fail or old technology would die.

With something like digital cable on tv you can get direct results for
advertising since it reports back...radios don't

On the same note you have a radio...who knows if it's 10-20 people in
the room hearing it...what if it's a warehouse and you have two radios
on the same station? would that count as two?

If there's no advertising that means it either has to be listener
supported (public radio), religious (waaay too many to list) or
government supported (VOA).

I really don't get how something that is mostly OTA can realistically
provide the numbers of listeners. I'm not debating as to what is and
isn't popular but the magnitude.