It seems it is pretty simple. Nobody buys anything anymore, really, and why should they? There is no incentive.
Soon Spotify will be it and if your songs are good, I guess you stand just a good a chance as anyone.
Self promotion is the game now. It's all about social media, pure and simple.
Yes, just having material on spotify (or iTunes, amazon, etc.) means nothing. The chances of someone randomly finding one of your songs amongst the other 30 million songs there are slim at best.
Self promotion is tough, how do you make yourself stand out from everyone else out there who is also plugging their songs?
I have quite a few people subscribed to my mailing list over on reverbnation, even more on my band page over on facebook. But the amount of people that actually read the emails I send or the posts I make on facebook is a very small percentage.
You have to have some decent marketing skills to make it happen I think.