Only in internet radio.

I have a theory that the reason why FM radio stations often hold contests is so that they can gauge how many people are listening. By counting the number of entries and calculating an estimate of what percentage of radio listeners enter their contests, you can get a rough estimate of how many people are tuning in.

Kind of like how Nielsen operates. Send out a survey to a percentage of the American population, and from their answers you can figure out how many people watch a certain show.