Open the Facebook app on your iPhone
Tap your profile picture (or menu icon)
Scroll down and tap Settings & privacy
Tap Settings
Tap Accounts Center
Tap Personal details
Tap Account ownership and control
Tap Deactivation or deletion
Select Delete account
Choose a reason (if prompted)
Tap Continue to account deletion
Enter your Facebook password
Tap Continue
Tap Delete account to confirm
Wait for the deletion confirmation message (you may need to log in again)