Griz,
The Thank You page is something you create. Does
site.com/thankyou work on your site? If not, you'll want to set that URL to be a full URL that does work. Generally, this is a link to a Joomla article that you use to "thank" the user for inviting friends, and should look like
site.com/index.php?option=com_content&view=article&id=12 (non-SEF) or possibly
site.com/thankyou (SEF with a menu item created for it).
In short, JFBConnect doesn't have a built-in Thank you page, you'll need to create that on your site.
Hope that helps, but as always, if you need anything else, just let us know!
Alex