If you're using it for hours a day, and not using many other apps, that's possible. (It's not 31% of your battery, it's 31% of the total amount of power being used from the battery - so the app used most will usually use the most power, although that would usually be the screen. and if the screen showed the actual percentage used any time any app used any power [it only shows whole numbers, so 0.4% would be 0], it would add up to 100% no matter how short the time, how much power was used or which apps were running - 100% of the power being drawn from the battery was being drawn by something. All of something is always 100% of it. the total amount of battery capacity Facebook may have used could be as low as less than 1% - almost nothing is using any power, but of that little bit being used, Facebook is using 31% of it. Don't use the percentage numbers as an indicator of how much power is being used, just as an indicator of which app or function is using what part of it.)
Charge the phone, then let it sit, unused, for at least a few hours (overnight is fine). Then look at the battery in Settings, press the graph and you'll get a second screen. At the bottom of that screen is a line labeled Awake and another one labeled Screen. When the screen line is dark, the Awake line should be almost completely dark too. (Some things will wake the phone once in a while, giving you very thin lines widely spaced.) If the Awake line is solidly on, something is keeping the phone awake. It could be Facebook, if you keep the app running, but if not, the shortest refresh interval is 30 minutes, so you must be downloading tons of posts. You're really popular. (I used Facebook a lot today, and it used 7% of the total used.)
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