I agree that there are TOO MANY ADS. I just tried Spotify. I have had several sessions with one song then 3-4 ads. I could not really believe that Spotify would really be giving so many, so I started counting. One time I counted 6 ads in a row. In the last 2 days I have not had more that 2 songs in a row.
So if you work for Spotify - I like your product, but I am going back to Pandora. Their free radio has less ads and less intrusive ads.
The above history lists can be manipulated if you chose not to listen to the whole song. I get the feeling that the frequency ads are played is based on the amount of elapsed time of music you chose to listen to. It appears to be approximately every 8-10 minutes of music played.