Lol this reminds me. A year back or something I used to be addicted to searching for music distributing sites; so I made accounts a bunch of weird sites which I even forgot their names lol.
Anyway, YouTube's community was awesome to me (I guess it's just your luck)! YouTube has a great community if you know where to look, and you get a lot of casual listeners and dedicated fans. Also the visual aspect to it is really good; it would be awesome if Newgrounds allowed you to upload a video to go with a song!
Soundcloud, as others have said, has a great commenting system; the only downfall is that artists begin feeling that having more comments = better music. Not very useful for beginners, really difficult to get attention.
Reverbnation... I'm not even sure why I still use that, it's completely pointless. It does have an advantage though, which is that many "big guys" use reverbnation to find upcoming talent, so if you're talented then use it!! :D
And finally, Newgrounds. For me, Newgrounds is just a community of artists (NOT listeners) where we can help each other improve and where we can give each other constructive feedback. If you're inactive here then your simply not known. Whereas a few years back it used to make people "popular" (such as dimrain), I feel like it's just become a hub for producers and composers to discuss topic (like this! :]) Also, yeah if you get your music into a game then you're all set for more attention :p