Merge user account

Merging user accounts (whatever that would mean…) is frankly beyond the scope of this plugin.

I suppose manually associating a Facebook ID with an existing user account would be a more accurate description.

Allowing existing users to explicitly associate their facebook account with their WP account is already a feature of the premium addon; see https://www.justin-klein.com/projects/wp-fb-autoconnect/#premium, 5th bullet.