Can we....

That would have to be a custom modification to JFBConnect. Since Joomla doesn't have any method to charge on registration or coupon system built in, we'd have to customize that to whatever extension you were using to 'charge' your users. JFBConnect is more focused on the social interactions between your site and Facebook and the single-sign on capabilities that Facebook Connect allows.

If you have a developer, we could easily walk them through how to determine if a user is logged in via Facebook (vs just through Joomla), and then they could use that information to offer whatever incentives you'd want.

Yes, it would be possible with Virtuemart. However, that would require some custom development. We can help, but wouldn't do it directly as we're currently booked for client work for the next few months. As with all of our extensions, we'll support the features of our extensions here in the forums. If you have a developer, we can gladly answer any questions about how to detect a Facebook user, but they would have to add the functionality to do 'something' with that knowledge.