Micah Singleton, writing for The Verge has managed to obtain a copy of the contract between Spotify and Sony Music from 2011, the time at which the streaming service arrived in the US. It makes for illuminating reading.
The article goes into much detail on the contract and the flow of cash from subscription and ad income and makes it clear where artists against streaming should be directing their anger. Short answer it isn't Spotify.
Music publishers certainly get their slice of the pie. How much of that they then pass on to the artists remains in question.