Facebook's app has long been a source of great discontent among the Android faithful. Facebook goes its own way with design and is slow to change direction, but it looks like some users are getting an updated version of the UI that has at least a little material influence. Specifically, there's a floating action button.
This is far from a full material overhaul—it doesn't have a tinted status bar on Lollipop. The addition of the FAB does at least get rid of that new post bar that floats at the bottom of the screen in the current app. Tapping the FAB brings up the post, photo, and check-in options. It's a much better use of space.
The material-ish UI is probably a server-side change, so there's no APK you can download to enable it immediately. In fact, you might never see it at all. Facebook experiments with a lot of different UIs, so there's no guarantee this will roll out to everyone.