One caveat to FBJS, however, is that most 3rd-party libraries are incompatible, which makes things like animation a little bit tricky. So we decided to create an animation library for FBJS to help developers out. Unsatisfied with the state of 3rd-party animation libraries, we built our own animation library from scratch. We've been using this new animation library internally for several months for animations around Facebook, and everyone here has been very happy with it. Now we're ready to release it to developers on Facebook Platform so you can use it too.
And then we thought: "why stop there?" The library had very little Facebook-specific code, so we took some time to modularize it, and now we're open-sourcing it for everyone. The code is released under a modified BSD license, which means you're free to use it pretty much wherever you want.
So check out the documentation and enjoy! http://wiki.developers.facebook.com/index.php/FBJS/Animation.