agreed - i think they could make considerable $ on affiliate traffic with iTune/Amazon if they improved the process to bookmark songs and purchase later
I don't know any start-up making significant $ from mp3 referral sales. Tops they may make couple grand but that is not much when you are getting 5m uniques(won't even pay for the streaming; and making 2k from mp3 affiliate sales is tough even with 5M uniques.)