I've had to figure this out twice in recent months, and it was no easier the second time than the first.
If you reinstall the Facebook app on Android, you will be plagued by the phone buzzing every few minutes to notify you that someone posted something.
The relevant setting is buried deeply.
- Open the Facebook app
- Click the gray-on-white hamburger icon. Top right, second row.
- Scroll all the way down to App Settings. Click.
- Click Notifications. It's not obvious that it's clickable.
- Change the Vibrate setting to OFF
- Curse Facebook.