From “FBJS/Animation – Facebook Developers Wiki“:

FBJS exposes a powerful animation library which gives developers an easy way to improve their user interface with a line of two or code. All animations are CSS based, so a working knowledge of CSS will really help you out here. An open-source version of FBJS Animation that will work on pages outside of Facebook is available at: http://developers.facebook.com/animation/.

Via ReadWriteWeb and the Facebook Developer blog.