New features of the app include support for high resolution photos, the ability to share posts, pull down to refresh, and support for Facebook Timeline (when viewing profiles). The overall performance of the app is similar to the existing release version, but the new design does make things feel more fluid and there's a greater sense of "Facebook familiarity" due the shared design langauge with Facebook.com and other mobile apps.
From the Windows Phone Blog:
Today we’re launching a new program designed to help speed up delivery of new features in the official Facebook app for Windows Phone and need sharp-eyed, energetic volunteers to download a beta version of our next release and tell us how to make it better.
Don’t like it when apps crash? This probably isn’t the program for you.
If, on the other hand, you want to see where we and the Facebook team are headed and want to provide engineers feedback during a time when it actually matters, you can grab the beta app here.
The Facebook beta app will not appear in searches or when browsing the Windows Phone Store (i.e. it is effectively hidden), but it can be downloaded by any Windows Phone 8 user by using the direct link. The beta will co-exist with the current version (4.2.1) of the main Facebook app, so you can have both installed on your phone at the same time.
Windows Phone Store description:
Facebook for Windows Phone makes it easy to stay connected and share information with friends. Thanks for being a part of our beta and helping make Facebook even better on Windows Phone. Along with all the great new features, there could be some bugs. Help us build a better app by sending feedback via Settings > About in the app.
Facebook Beta is a free download from the Windows Phone Store.