It's a good question. What I know is the quantification of audience is different. While Nielsen will show streams of on-air stations, the posted numbers don't come near what station data indicates. Last week Beasley Broadcasting said their online listening accounts for 40-60% of their audience. That's not what Nielsen says.
The majority of AM/FM listening continues to be over the air (89%) as opposed to streaming (11%), according to the latest Edison Research numbers. Yet as radio continues to evolve
www.insideradio.com
But if what Beasley says is true, then there's money to be made with online radio.