At Facebook they are just a team of idiots with a messy policy.
Facebook app doesn't have dark mode. WhatsApp doesn't either. But Messenger do.
There's no Facebook app for Watch. There's no WhatsApp app either. But there's a Messenger app.
The iOS app and the mobile website doesn't let you use the full pic without cropping when uploading a new profile pic. The website does.
You have iPad versions of Facebook and Messenger but no WhatsApp.
And I could swear every single update pushed for the Facebook app contains cosmetic changes in the interface (like the icons, constantly changing).
It looks like those guys act like 18 years old "designers" that just discovered stuff and make changes all the time.
I know I don't answer your question, sorry, but that's my feeling about the situation and therefore I stopped expecting a dark mode or anything else from them... It will come when it will come.