Generally, what you're seeing is one of two things. One is from Facebook's API, which is just Facebook comments in an iframe, essentially. The other is often a Disqus plugin.
Technically, vB already allows this if you tried to comment and your taken to a login page with the Facebook login button there. But that's less evident to a random visitor. Only thing I could think of is to include an FB login button near or around Quick Reply that's only visible to Guests. (Then again, I don't recall if QR can be made visible to Guests without allowing Unregistered Users to reply.)
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