I'm unsure. Does the page load normally when it's not in preview mode? I'm unsure what the preview mode is or how it works, so it's difficult for me to know what may be going wrong there.
I see in the inspector that something wrong seems to be happening right after our script tag, but the text there doesn't look to be from JFBConnect. I'm not sure if JFBConnect is the cause of the problem or confusing the preview mechanism somehow. You may want to contact the JoomlArt team to get some details on what they think could be happening.
Alternatively, you can try to disable the JFBCSystem plugin whenever you need to use the preview mode for testing or whatever it's for and then re-enable it when done. That's a temporary, not great, work around, but hopefully it helps.
Thanks,
Alex