The beta version of Facebook is used by Microsoft to test out changes to the code before they are included in the regular Facebook application a few weeks later. Consider this a heads-up of what's coming next, which means you will have more functionality, but risk having an app that may crash or be a touch temperamental.
That's not for everyone, but it's a nice move to have the final stages of development as public as this.
Facebook beta 5.2 added in improved Live Tile support.
The current beta release builds on the major changes added in the 5.2 release (detailed here). While there are no fundamental additions in this point release, the speed and performance of the beta have been improved. This is where the beta is useful. These performance changes can be tried out on a wide basis in the real world, and if they cause issues it is not the end of the world. Once everything is stable, we'll see the main app updated with the same changes.
If you're happy to go for the beta, head over to the AAWP App Directory to download the latest version.