I have no idea why you're seeing that. The activation link looks correct. It's going to the com_users component, as it should. I'm not sure what's denying the users component from operating, but I don't think it's a JFBConnect/SCLogin issue.
Can you try registering a new Joomla account the 'standard' way, without using a social network, and see if the activation link fails then too?
As for causes, I'm really not sure. My only guess is that some plugin or security extension is preventing access to that page for some reason. To eliminate JFBConnect as a cause, you can disable the JFBCSystem plugin.
Beyond that, try to set "Debug Mode" to Yes in the Global Configuration area. That may have more information on the error page.
I hope that helps narrow things down, but keep us posted if there's anything more we can help with.
Thanks,
Alex