Facebook suddenly won't stay in dark mode and aaarggh, my eyes! - Mashable

Following the latest app update on iOS, my Facebook app keeps reverting to light mode, no matter how many times I set it to dark mode.

It appears you've also broken dark mode, Facebook.

It can be set to "On," "Off," or "System," which checks whether dark mode is enabled on the iPhone or not, and adjusts the app's appearance accordingly.