Hrmm. Not sure why that's cropping up now. However, there should be an easy fix, if you wouldn't mind trying it.
In the /plugins/user/jfbconnectuser.php file, you'll see:
jimport('joomla.plugin.plugin');Add the following line directly below that:
jimport('joomla.filesystem.file.php');
Let us know if that fixes you up. We're releasing JFBConnect v4.1.1 tomorrow, and this fix will -not- be in there, so you'll need to do this again when you update.
Sorry for the trouble, and good luck,
Alex