This happened to me too, and I was like, "why facebook keeps logging me out after i closed the app??" Turns out, my phone’s battery saver mode was messing with the app. If you’re on Android, try turning off battery optimization for the Facebook app.
Go to Settings > Apps > Facebook > Battery > Battery Optimization and set it to "Don’t optimize." Worked like a charm for me!
Go to Settings > Apps > Facebook > Battery > Battery Optimization and set it to "Don’t optimize." Worked like a charm for me!