Facebook app for Windows Phone 8.1 devices just got updated to v126.96.36.199 in Windows Phone Store. Unfortunately, this is also a minor update with no new features. This update should include few performance improvements and bug fixes.
If you have installed the Windows Phone 8.1 update, this Facebook app allows you to enjoy OS level integrations such as in People Hub, Calendar and Photos app.
The Facebook app for Windows Phone can be updated in the Windows Phone Store here.