Musicians like myspace because you can customize your homepage. And it's accepted to send invites to random people. On Facebook you don't have either of these. Also they added music friendly features such as letting users select your tune as their homepage theme song. As a musician I've found Myspace to be invaluable.

I think they face stiff competition from soundcloud. But a big problem with soundcloud is the amount of spammy bands that comment on songs and the lack of casual listeners like on myspace.