While many people are not on their smartphones from the beginning of the day to the end, many still end up going through plenty of data each and every day which translates to a lot of data usage every month. For those who have truly unlimited data plans with no throttle limits, this isn't much of an issue, but not everyone has unlimited data and could end up breezing through their 4G LTE speeds rather quickly. Apps like Netflix have introduced ways to save data automatically for users by letting them set limits on the quality of the video stream, and now Facebook appears to be doing their part to save people data as well with a new toggle switch that has been seen inside of the Android app.
The toggle is only showing up for some, so it's likely that Facebook is testing this with just a small batch of users at the moment, with potential plans to roll it out to a wider audience later on. To see if you have the feature yourself, head to the right sidebar menu within the app and if you see a "use less data" option showing at the top, which is where it should be located if available, then you can tap on that to enter the setting and enable the toggle switch. Likewise, this is where you would return if you wanted to turn the toggle off as well.
Clearly labeled in the description, the use less data option simply reduces photo size so that the app has to process less data when rendering pictures in your news feed for you to view. It's a neat little trick that isn't too complex, although it's hard to say how much data it will actually save people in the long run, and Facebook doesn't appear to have any explanations for it, yet anyways, which sort of makes sense given the new toggle hasn't officially been announced. Whether or not Facebook decides to roll this feature out to all users, if they even decide to keep it, is still unclear, and if it does launch it may not end up in the same spot as it is now for those testing it out.