I just tried to register with Google+ and my account was created automatically, as expected.
Can you tell me what process you're going through for registration? If you're not using a social network provider from JFBConnect, then the error can't be caused by us.
With that said, Session Token errors are usually caused by caching issues. I'd recommend disabling all caching in the Joomla Global Config area, the System - Cache (or Page Cache) plugin and any other 'optimization' extensions you may have enabled. If that helps fix things, then start re-enabling them one by one to narrow down the problem.
I hope that helps get you started,
Alex