Gui,
Sorry for the delayed response. We've heard of this on one other site, and aren't sure what's causing it. We are investigating, but don't have any great answers right now. The reason it's happening is that Facebook is not properly deleting the user's cookie with their FB User ID. JFBConnect uses that cookie to automatically log the user in, which is what you're seeing.. even though the user isn't actually logged into Facebook at the time.
The best thing I can recommend is ensuring that your Facebook Application's Site URL and Domain URL are absolutely correct. For your site, they should be:
Site URL:
n-pix.com
Domain: n-pix.com
If that's correct, we'd recommend disabling the Auto-login functionality for now. It's likely a Facebook Javascript issue that's affecting some sites, but it's not widespread, and we haven't found a bug report for it yet. Facebook releases updates to their Javascript on Tuesdays, so it's possible it will just be fixed tomorrow.
Hope that helps, and definitely let us know if the issue changes for the better or worse.
Thanks,
Alex