I would speculate that this could be something like http://turntable.fm built into Facebook.
If so, what a great way to keep users actively using Facebook: I can imagine someone spending a lot of time doing other things on the site while listening to music with their friends.
My senses point to it. From my perspective it makes sense because you can't really point to it being a "serious" app that [serious] matured users would point to as a sense of belonging and we all know that Facebook users have no embarrassment at all when it comes to publicizing anything and everything they deem worthy in an effort to appear cool and belonging.
If so, what a great way to keep users actively using Facebook: I can imagine someone spending a lot of time doing other things on the site while listening to music with their friends.