Facebook is currently trialling this on the Android beta version of the app. It works by changing how the app downloads any media content the user receives, such as photographs and videos.
Here’s how it works. Without the Data Saver mode active, Messenger automatically downloads anything the user receives. This happens regardless of the size of the content, or whether the user actually wants to see it.
When ever this feature is on, you have to tap-to-select each individual photograph and video you wish to view. As you might expect, ordinary text messages are unaffected by this.
The beta also comes with a counter, which shows the user how much data it has saved. This can be reset as required, which should be helpful to those on a rolling monthly data plan
The Data Saver features only works when the device is using cellular data. When it connects to a Wi-Fi hotspot, Messenger reverts back to normal.