Hi Michael,
You didn’t do anything wrong, enabling WP_DEBUG mode in wp-config.php file is very useful in case you are investigating a problem on your website, with that mode ON, you got this notice regarding Undefined index share, actually after further investigation, this error is a symptom for another error, which is:
Application request limit reached from Facebook API, so I recommend creating a new Facebook application ID/secret following these steps:
then replace the ones you have currently in (X > Facebook Comments) with those newly created, this should resolve the main issue, regardless WP_DEBUG mode is turned ON or OFF.
Thanks.