I think Neil Young said something along the lines of "I look at radio as gone … Piracy is the new radio, that’s how music gets around."

He uses the word piracy which doesn't apply here, but I think what he says is relevant in this context: a lot of people hear music for free, like it, and then buy it. Maybe they hear it for free on the radio, through torrenting, or through youtube. Although the radio and torrenting does generate revenue as well as promotion whereas torrenting only does promotion.