Why can't I log into the Facebook app on my iPhone?
If you're having trouble logging into your Facebook account from your Facebook app: Make sure that you have the latest version of the Facebook app, or delete the app and then reinstall it. Try logging in from a mobile browser (example: Safari, Chrome).
Why can't I log into the Facebook app?
Check For Updates (Android and iOS).
If you cannot access your account on the app but can log in on Facebook's website, then try to update your Facebook app. Possibly, the version of Facebook installed on your device is no longer supported, or it contains a bug or glitch related to logging in.