A Facebook application would be HUGE. We would double the amount of users literally overnight, guaranteed.
I also have an idea for the company to help recruit new members. They should offer current users the option of joining a "street team" to get the word out about the site. If they sign up for the street team, send them some flyers or promo CD's with a special passcode on them. They could pass them out at local concerts or a college bulletin board. When new people are referred to the site, have them enter the referral person's passcode they were given to show they recruited someone. For every X amount of people you recruit, you get free song downloads. The user wins by getting free music, the company wins by getting more users.
would be a great idea that could propel this into space if the company would be willing to do this.