Samuel Gibbs, writing for The Guardian:
I accessed Facebook for the same amount of time, and for the same purposes, using the social network’s excellent mobile site within Safari, as I had done using the app. I also left the Facebook Messenger app installed. […]
To make sure that this wasn’t an isolated incident, I also recruited several other Facebook-using iPhone owners to conduct a similar test. They all found similar results, with increased battery life when using Facebook in Safari having uninstalled the main Facebook iOS app.
If you feel that you need to continue using Facebook, I suggest uninstalling the app and adding a website shortcut to your iPhone’s home screen.