Make sure it's good before you going YouTubing and Facebooking. Last thing you want is a bunch of folks saying your friend's music blows. Also, there may be copyright issues at stake, so be wary and know for sure your buddy wants the assist. Some folks don't like other people spreading their stuff around.
After you've got those things down (YouTube, Facebook, Twitter), try hitting up appropriate markets for the music - I'm guessing CAG isn't a big hip-hop/rap nexus, but there have to be sites specializing in that type of music. Find those and write the editors, ask them to listen to the song(s) and give props on their site, then back-link to those sites on YouTube, Facebook, etc., so they get traffic back from other viewers. Tell them you'll crosslink.
Should keep you busy for a while.