How to Delete Facebook Account | Permanently Delete Facebook On Android Device | Completely Delete My Facebook Account Forever: When a Facebook account is deactivated, it can be restored by simply logging in with your Facebook user login details.
See: How Do I Delete My Facebook Account
You can deactivate your Facebook account in case you want to get rid of Facebook temporarily. On the other hand, if you want to leave Facebook forever and do not want to come back, you can delete the account permanently.
The Facebook app that almost all Android smartphones have only allows the users to deactivate their accounts. In order to delete your Facebook account permanently using your Android mobile phone, you must consider using the mobile version of the web browser instead of using the conventional Facebook app, and after signing in to your Facebook account from the browser, you must use a particular URL that will redirect you to a new page from where you can permanently remove your account profile off from Facebook.
Here is how:
Turn on your Android device.
Tap the Menu button to go to the apps list.
Open any of your favorite web browser from the given list of apps.
After opening the web browser, open Facebook website on a new tab.
On the opened window, provide your login credentials to sign-in to your Facebook account.
Once logged-in to your Facebook, copy or type https://www.facebook.com/help/delete_account in the address bar of the web browser.
On the displayed page, tap the Delete My Account button under the Delete My Account section.
Once done, on the Permanently Delete Account box, enter your Facebook password in the Password field.
In the Text in the box field, type the captcha code displayed in the image.
10. Finally, tap OK to permanently delete your Facebook account.
Source: Delete Facebook Account