Hi,

although technically this would possible, it's really not doable because of the way that Joomla manages passwords in the database table.

This would have the side effect to overwrite the password chosen by users with a random generated password used by the Facebook Login to login into the system.

Joomla encrypts password so it's not possible do a login with an existing user account programmatically. For this reason the Facebook login has to be used by users that have not already a standard account in the system or at least ensure that a new account can be created having not the same email, because Joomla also requires unique emails address in the system.

Best regards

John