JFBConnect has it's own registration process that needs to be followed to ensure that the user's Facebook account is properly connected to their Joomla account (and that that account is created if necessary first). There's no way to use the standard Joomla registration form with JFBConnect as we can't control that process like we'd need to do to properly setup the Facebook/Joomla connection.
We don't test directly with JSPT, though we know many users have got that solution to work. You may need to make sure the JSPT User plugin is ordered after the JFBConnect and Joomla user ones so that it's properly seeing our new users.
We'll gladly help how we can, but I'm not terribly familiar with JSPT. We currently integrate with JomSocial's built-in multi-profile system though. Doing a search though, there are multiple posts on JoomlaXi's forums looking for information about integration between the two, such as:
test.joomlaxi.com/forum/61-jspt3-5-xxx/2...-payplans-jfbconnect
We don't currently have a subsrciption though, and can't see what the response there looks like.
I hope that explains a little bit, though I know there aren't any great answers.
Let us know if you find anything in the Joomlaxi forums or whether the plugin ordering mentioned above changes things.
Thanks,
Alex