I wouldn't be surprised if they start seeing legal trouble soon if it hasn't started already. I would gladly pay money to use it, if it comes to that, as I haven't had this much fun with a site for a very long time.
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.)