It comes and goes all on its own. I will have dark mode for a few days and then launch Facebook on my iPhone and all of a sudden its light again. I haven't done anything to cause this to happen and there haven't been any updates from the App Store that would have prompted this either.
What I find helps bring dark mode back when this happens is to swipe up and terminate Facebook and launch it again. But I typically have to do this multiple times. The first few times I launch it, it still comes up in light mode. After about the 3rd or 4th launch (after terminate it each time), it finally launches in dark.
This may all be psychosomatic on my part. Perhaps if I just left it alone, it might revert back to dark mode on its own after a few minutes. I don't know because I don't have the patience to read Facebook in light mode due to vision issues. I do not normally terminate apps but I make an exception in this case for Facebook.
The way they have implemented dark mode makes absolutely no sense to anyone but them.