After just four years in America, Spotify is already as big as Pandora | Fusion

I can't say this is surprising. From my perspective, Spotify has the greater value proposition since it offers both radio and the ability to build your own larger music library. 

The thing Pandora has going for it is how entrenched it is in the habits of consumers. But as new users enter their market, they will ultimately be better served by something like Apple Music, Rdio, or Spotify that offers more than just radio stations.