Alfredo,
In the JFBConnect Requests area is a log of every request that has been sent from your site. That will always include the Facebook User information and can include the Joomla user info, if they are logged in when they send the invitation. With that, you should easily be able to tell how many requests a user has sent out.
I hope that helps answer your question, but if you need more info, just let me know.
Thanks,
Alex