Best way I have found is marketing it on forums where many people share links such as mp3s and movies. Once one person uploads something then many will use that same file sharing service and after awhile if your service is good then everyone on that forum will use it. I have seen this happen many times on forums such as Boxden and Real Rap Talk which really don't even have music it is more for file sharing. The thing is most of the websites disappear within a few months due to legal reasons or just not enough profit margin and if your page isn't monetized correctly then you lose money.