Session Error Showing Up In Debug Mode

The admin bar disappeared on the front end of this site so I enabled debug mode. I get two warnings. One about a deprecated MYSQL call, and this.

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/XXXXXX/public_html/wp-includes/wp-db.php:1518) in /home/XXXXXXX/public_html/wp-content/plugins/wpmu-dev-facebook/lib/external/facebook.php on line 60

Not sure if this is related to the admin bar vanishing, but guessing it is. I'm also seeing this on another site that is having issues.

Any guidance would be awesome as I'm not sure how to resolve this.