Hi and welcome to Apple Discussions...
Quit Safari.
Open Keychain Access (Applications / Utilities) Select Passwords on the left.
Delete the Facebook keychain.
Relaunch Safari. Navigate to the Facebook page. Login with your user name and password, click Yes when prompted.
If you still have problems, delete the "Safari Forms Autofill" keychain. Relaunch Safari and try again.
Everything is checked, except "user names and passwords".
That must be checked if you want Safari to save your Facebook user name and password.
Carolyn ๐