Can you let me know the URL (you don't need to include the domain) the redirect goes to? Does that page work normally if you go to it directly (without authenticating with Google/JFBConnect first) or does it always fail?
There isn't a way to force the user to fill out additional information within JomSocial if they register with JFBConnect though. However, there are some 'Profile Completeness' plugins for JomSocial which may work for you.. they make sure the user has certain fields filled out properly before letting them use the site otherwise.
Also, there is a plugin within JomSocial called "User Redirect" (or something like that) that may need to be disabled as it's caused redirection issues in the past.. though it shouldn't be a 404 error. You may want to disable or look at the settings for that plugin to make sure it is setup correctly.
Finally, I apologize for the lengthy delay. We had an issue that was preventing us from responding to support requests but it is rectified now.
Thanks,
Alex