...Pandora and TargetSpot are probably the biggest two holders of digital streaming inventory. Clear Channel’s iHeartRadio doesn’t have any ads yet. And Spotify’s radio functionality is just that — one small function of its overall business, so the company sells ads more like digital units than like terrestrial radio spots. It’s a market that’s still young enough to be figuring things out. With a new owner and possibly new attitude, Arbitron could be there on the ground floor.