There's a few ways. In JFBConnect, on the Overview page, you can see the rough analytics of the amount of users that have engaged with your site recently.
To see the number of users that have linked a Facebook account to their Joomla account, you can go into the UserMap section. Each row in there is a 'connection'. That connection can either be someone who initially registered through Facebook, or someone who had an account and connected it to their Facebook account.
Hope that helps,
Alex