According to 9to5mac Facebook has confirmed that it will publically roll out Dark Mode for iOS devices iPad and iPhone. Several users have spotted the settings for dark mode already on the iOS devices. It was first reported in April that Facebook is bringing Dark Mode for its primary application on iOS. At that time it was not public.
At that time Facebook said that its Dark Mode for iOS will not be like Messenger and Instagram. Facebook said it would be a Grayscale design. The colors of logos and icons will be white.

Facebook also said at that time the users can manually enable or disable the dark mode. They can use the in-app settings pane to do that. Facebook also said that it will include the adjustment for dark mode based on iOS preferences.
Now several users have posted on twitter with images that show the Facebook app now has the dark mode setting.
One user, Matt Navarra on Twitter posted a photo of the settings for the dark mode saying:
Facebook appears to be rolling out Dark Mode… https://t.co/qSfDa8d51e
— Matt Navarra (@MattNavarra) June 26, 2020
The dark mode for Facebook is not yet universally available. It is only available for a small percentage of Facebook uses all across the globe.