That is an error message that will only show if the Error Reporting setting in the Joomla -> Global Configuration area is set to "Development", which is not what your site should normally be set to while in production. The message also only shows when running on Joomla 3.4, which was recently released.
With that said, please see the temporary workaround at
www.sourcecoast.com/forums/jfbconnect/jf...-to-joomla-34#p51478 which has some instructions to alter code if you want to fix it now (and not change the Error Reporting setting.
The fix has already been checked in for our next JFBConnect release.
I hope that helps,
Alex