The problem with a lot of these sites is, once everyone and their mother starts using them it gets way too crowded to do you any good in terms of marketing or promoting yourself.
If everyone's got the exact same advertising/distribution/networking tools...everyone does the exact same thing. That's not necessarily a bad thing until the volume of music seen on the page gets way too large to manage, and people stop wanting to sort through all the junk to find someone they like. You end up getting lost in the mix.
That's why pages like MySpace are now obsolete. The time and/or money you have to spend to get a positive and widespread reaction just isn't worth it.
Just my two cents.