The Official Facebook App For WP8.1 is different branch of the app

wp_ss_20140415_0004 wp_ss_20140415_0003

If you have installed the Windows Phone 8.1 update, don’t forget to also uninstall the Facebook beta and install the mainstream Facebook app.

When installing the app you will notice you are being offered a slightly different version,  a new 8.1 branch of the app, which offers integration with the People Hub, Calendar and Photos app.
The effect is most easily seen in the calendar, where Facebook Birthdays now show up.

The Facebook app for Windows Phone can be found in the Windows Phone Store here.