Can this be used to allow the Joomla login connect to a 3rd party site

No, JFBConnect can't do what you're looking for on it's own. JFBConnect ties into Joomla's underlying authentication system entirely. JFBConnect is meant to allow authentication from other sites to work in Joomla. It's not meant to be a single-sign on provider into other sites.

There are other extensions out there to do that for Joomla which do work with JFBConnect, but it's not something we can implement directly.

I hope that helps explain,
Alex

I had a feeling you were going to ask, and unfortunately, no I don't. JFusion did exactly what you're looking for and integrated with multiple other authentication systems. Unfortunately, development stopped on that earlier this year. You can still probably find it, but you won't get any support or development going forward.

There are other systems, like p8pbb which is just for integrating Joomla with phpBB, but that's very specific.. and I don't know what other system you're trying to work with.

Sorry I couldn't provide more details, and please do let us know if you find anything!

Alex