I'm unsure why you're getting that warning. Can you try the following:
* Go to the Configuration -> Facebook area and set an AppID and Secret Key of 1234 (just garbage stuff is fine)
* Go into the Profiles area of JFBConnect and simply hit "Save"
* Go back to the Facebook area and remove the AppID and Secret Key
That should re-store the profile field mappings in your database and, hopefully, fix things up.
If not, please don't worry. The PHP warning is just that, a warning, not something going wrong or causing functional issues. You can safely hide that by going to the Joomla Global Configuration area and setting the Error Reporting setting to 'None', which is recommended for production / live sites anyways.
I hope that helps,
Alex