This article explains how to get dark mode back in the Facebook app. Instructions apply to iOS and Android.
How to Fix Missing Darkmode in Facebook – iOS & Android
An outdated app is the most common reason why you can’t enable dark mode for Facebook. Follow these steps to get it back:
Close Facebook by swiping it off the screen.
Update the app. This works the same on both platforms: Android [Update Android Apps] and iOS [Update iPhone Apps].
