iPhone Facebook password

A couple of possibilities:

• If your iPhone X has the same Apple ID as another device, then it is possible the password would transfer over if iCloud Keychain is being used.
• It may be that the Facebook App was only deleted from the Home Screen and still resided in the App Library, so none of the information was actually deleted from the device.

607c wrote:

I changed my Facebook password on all my Apple devices with the exception of the iPhone X. On each device after deleting Facebook and reinstalling it would ask me to sign in with password and it was successful. When I deleted Facebook from my iPhone X and reinstalled it no such message appeared. Not sure if the new Facebook password was automatically transferred to the iPhone.

ask facebook.