I used this awhile ago, but I just now remembered it and I thought I'd share it with you guys.
Listenpledge! You hook up the website with your Soundcloud, and then you select a song to share, then eventually you get a song paired up with you and you comment on it, and another person comments on your song! It's worked out well for me so far, but I could see where it could be abused. Like I said in the title, it's like Sub4Sub in that you exchange your content for someone else's content, but it's actually good in that it forces the other party to partake in your content.
So check it out. Or don't. I can't stop you.