Amazon's growth is still noteworthy. Outside of Apple Music, it's hard to find any other streaming music services burgeoning at rates even vaguely comparable to Spotify. And that 70 percent relative growth is nothing to sneeze at -- it shows that Amazon Music is evolving from a convenient service for Echo speaker owners into something more broadly appealing. Unless Amazon can keep growing at a heady pace, though, a top dog like Spotify is unlikely to feel seriously threatened.