It couldn't be any easier to do so, you just swipe to the screen with the Facebook logo, hold a finger down on the app until it starts wiggling, tap the little "X" at the corner of the app (or tap "Rearrange apps" and then the "X" if you're running iOS 13), then hit "Delete" on the popup window to confirm the move.
Here's how, in quite a few steps, which can change from time to time with updates.
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1. Launch the Facebook app on your phone and then tap the three parallel lines at the bottom right corner.
2. Tap "Settings & Privacy" at the bottom of the list, and then "Settings."
3. Scroll down and tap "Account Ownership and Control."
4. Tap "Deactivation and Deletion."
5. Select "Delete Account" by tapping the dot beside that option, then hit the blue "Continue to Account Deletion" button.
6. Scroll down and tap "Delete Account" and then confirm the deletion with your password, then one more tap.
And don't worry, if you change your mind within 30 days, you can simply log into your Facebook account to stop it from being forever purged from the internet.