Facebook Beta app for Windows Phone 8.1 devices just got updated to v188.8.131.52 in Windows Phone Store. This is clearly not the update that Joe Belfiore mentioned during his AMA last week. This update only includes performance improvements and bug fixes.
Joe mentioned at that time an update will bring improvements to performance, having full-width photos in the newsfeed, and seeing photos & comments at the same time. We can expect that update in the coming weeks.
The updated app can be found in the Windows Phone Store here.