Facebook is one of them. In the meantime, Facebook is also reportedly working on a dark mode to the Facebook app on Android. There's no official word on it but a hint was dropped in the set of codes, found by a researcher named Jane Manchun Wong, in the Android app.
Wong posted a screenshot showing how the dark theme could look in the main Facebook app. By waiting, Facebook will be able to learn from the experiences of developers who have been quicker to create dark modes for their apps. Wong notes that only portions of the app have been revamped with dark mode thus far, meaning that you'll often see mishaps like dark text on a dark background.
However, she noted that this feature seems to be in an "early stage of development" - it's only appearing on the app's Groups tab and some dark text displaying against a dark background, making it tough to read.
Different users prefer dark modes for different reasons; some want it for because apps with darker backgrounds are easier on the eyes, some prefer the elegant aesthetics, while others just love to extend their smartphones' battery backup.
Facebook is working on a dark mode for its mobile app giving you an alternative to the current stark white design.