Logging into Facebook - Sorry, User ID not found.

Can you let me know a few things:
* Is this on a first-time registration, or on logging in with a social network on a returning visit?
* What social network is this happening with?
* Is this with every user, or just a specific one?

Another thing to check is to go to the JFBConnect -> Usermap area and search for the user that you are trying to authenticate with. Make sure that the data looks correct for the social network account and the local Joomla account. If the connection between those is off, or if no user is shown in the Joomla User column for some reason, that would explain the problem. You can also try to delete the mapping there (it won't delete the actual Joomla user) and reconnect the accounts on the front-end again.

I hope that helps narrow things down, but keep us posted on the questions and suggestions above.

Thanks,
Alex