You can permanently delete your Facebook account on the iPhone app. Once you’ve disconnected your apps and downloaded any data you want to save, then you can permanently delete your Facebook account using the Facebook app for iPhone.
Before you delete your account, you might want to take advantage of the other options on the page to save your posts or download a copy of your personal information. When you have saved any personal information you care to keep, tap Delete Account and confirm you want to do this.
What to Do Before You Delete Your Facebook Account
If you are intent on deleting your Facebook account, there are a couple of things you might want to do before you pull the plug once and for all. Specifically, you might want to save a copy of your personal data, which includes posts and photos. Perhaps more importantly, you should disconnect any other apps, websites, and services that use your Facebook credentials to log in.
For more information on how to delete your Facebook account on iPhone visit Lifewire.com. There you learn how to only deactivate your account if you would want to use it in some other time.