Apparently, you will not see the Facebook data saver setting from the latest app update. However, Data saver settings were still available in some devices which are not using the recent update.
To enable data saver from your Android Phone, Tap the Hamburger key or Settings, scroll down to Settings and Privacy, and Click Data Saver.
What is Data Saver in Facebook? Enabling this option will reduce the use of data when playing videos at low quality or medium resolution.
How do I limit Facebook data usage now? To do this, Watch tab then choose any video to play. Tap the video to see the gear icon or settings. There you will find the Data Saver option. Tap it and enable data saver, otherwise tap optimized.