This seems like a roundabout to go about finding the useful data. If a few others could just open Settings > Battery > Last 7 Days, then tap on Facebook, I'd like to see your numbers.

48 min on screen, 13 min background, 2% of overall battery

Note: Background App Refresh for Fb was set to off!

The ratio of background time to screen time is high but Facebook isn't the worst offender. For example:

Instagram - 5.8 hrs on screen, 1.5 hrs background, 18%

Snapchat - 57 min on screen, 1.4 hrs background (whoa), 3%