Hi, the locale recognized by Facebook is not necessarily the same as the WordPress locale. This is the case in Finnish, where for years we have used 'fi', but Facebook only recognizes 'fi_FI'. This results in an error with the FB debugger. I know you can't take into account every single locale and exception in your plugin, but it would be nice if you could add filtering capabilities to the locale function in the WPSEO_OpenGraph class instead of just echoing the WP locale.
Any other suggestions how to fix this are welcome of course.