MP3.com and SoundCloud are not as comparable. SoundCloud has really re-defined how new music is released and how new artists promote their music.
For promotion Soundcloud undoubtly serves its purpose, but actually releasing the music has to happen somewhere else. And in my opinion the actual re-defining service in that regard is Bandcamp. After all, you can't buy music on Soundcloud, can you?
For some reason I assumed there was a buy link on the preview tracks people put up.