Facebook has added a basic data saver option to the Messenger application. This should be useful to anyone looking to save some data while out and about outside of Wi-Fi coverage.
Once enabled, the data saver will show a click to download option for images and videos sent to you, so you can manually download them instead of the app automatically downloading them for you. If you're on Wi-Fi, the app will automatically download them as usual.
The feature seems to still be in testing phase, and may not be available to everyone at the moment. Keep a look out for when it eventually shows up on your phone.