Facebook Dark Mode

See this support article:--> Use Dark Mode on your iPhone, iPad, or iPod touch - Apple Support

All apps supporting dark mode, will show that feature.

Regards

Giulio

I don't use Facebook, so I can't help you any further. I don't even know if FB now supports dark mode (it didn't, till the end of 2020).

Should be this case, contact Facebook support team and ask them.

If dark mode works for other apps on your iPhone, it's not a problem related to your iPhone general settings.

Regards

Giulio

Use this link to contact an app developer:--> How to contact an app developer - Apple Support

or join one of their forums and ask there.

Regards

Giulio