Apple's iTunes is the largest retailer of music in the universe. But if they're not careful, that's not going to matter for much in the not-too-distant future.
There's a slow but inexorable movement towards widespread adoption of music streaming services. After years of grappling with the idea of "renting" music--i.e. subscribing to an all-you-can-eat service like Rdio or Slacker for $10 a month--people are starting to figure out that this is a very cost-effective way of consuming music.
Yes, there are purists (like me) who will insist in the comfort of owning music, whether it be a digital file, a CD or vinyl. But for the many times I have a transient interest in a song, an artist, an album, a scene or a genre, I'd rather just "rent" this music through a streaming service rather than spend substantially more money (and time) acquiring it physically. Besides, my wife is telling me that if I keep collecting music at this, we'll have to move again in 18 months.
Apple may sell a gazillion music files now. But if they're going to maintain their market-leading position, they're going to have to transition at least a portion of iTunes to the streaming world. And that means figuring out a way for subscribers to share music.
(Ping? Please. Let's not bring up that disaster. It. Did. Not. Work.)
The Harvard Busines Review has a rather interesting opinion on the matter. It's worth reading.