Facebook starts rolling out new UI on Android devices1
Facebook hasn't yet made any official announcement regarding these changes, but many users have already taken it to Reddit to highlight the new UI that's rolling out to Android devices. The most obvious change (and possibly the only one worth mentioning) is the relocation of the tabs from the upper side of the app to the bottom.
With the tabs now positioned at the bottom, Facebook is becoming so much easier to use with just one hand. Having all six tabs placed on the upper part of the app made it impossible to use Facebook with just one hand, so the new changes are extremely useful.
Unfortunately, it seems that Facebook is still testing the new UI since some users report their Android apps reverted to the old look shortly after they noticed the new changes. Some still have the new UI, so there's hope for those who haven't yet received the update.