Juser warning: Unable to load users

That is bizarre. The only reason I can see that happening is if you deleted some users from Joomla while the JFBConnect - User plugin was disabled. That plugin is what detects a user deletion and removes the entry from the JFBConnect database for that user. Does that seem possible?

I believe JFBConnect is smart enough to automatically delete those entries when you enter the Usermap area as well... so you should only see them the first time the page loads and tries to fetch the invalid connections. That's not something I've tested recently, but may solve the issue and make the problem go away after a few loads.

If none of that helps or explains things, please let us know. We'll gladly help investigate further.

Thanks,
Alex

I have just installed JFBConnect a few days ago after removing it more than a year ago.

That likely explains it. When you uninstall JFBConnect, we still retain some information in the database. So, while JFBConnect was uninstalled, if you deleted any users or made some changes that weren't synced up in the tables from JFBC that remained, then installed again, things would have been out of sync.

Are you still seeing the errors, or did they clear up after a few loads?

The list of users under User Map does not show all users that I have in Joomla...

This is normal. Only users that have connected their account to a social network would appear in the Usermap area.

Thanks,
Alex