Facebook's apps will update automatically even when you turned off auto update in play store.
How do I know if Facebook is updated?
https://www.youtube.com/watch?v=PvBe6pte2EQ
How often does Facebook app update?
twice every day
Why is my Facebook not updating?
If your Facebook feed doesn't appear to be showing the most recent posts, or if some posts which are shared to your Facebook page are missing, then the most likely explanation is that those posts in your feed may be shared from a user's personal Facebook profile or a Facebook page which has an age or location ...