JFBConnect uses whatever is set within Joomla for the Description and Title tags. For the description tag, for instance, we call:
$doc = JFactory::getDocument();
$desc = $doc->getDescription();
That's asking Joomla what the description for the current page is.
There are only 2 reasons I can think of for the tags for sh404SEF to not be used:
1) They're changing the tags after we ask Joomla for what they are. To fix this, please go into the Plugin Manager. In the 'system' group of plugins, make sure the JFBConnect plugin is ordered *after* the sh404SEF plugins. That should make sure sh404SEF does its work before JFBConnect tries to do anything.
2) sh404SEF is going outside of Joomla to set the tags in some other way.
My guess is it's #1 because we haven't heard this issue before and many users use both JFBConnect and sh404SEF.. but let us know if #1 fixes things or not and we'll continue to help however we can.
Thanks,
Alex