Is this the link for activating the user's account? If so, there are a few things that could be wrong and to fix it.
First, if you don't want social users to have to activate their accounts, you can enable the "Skip CB Activation" setting in the JFBConnect -> Profiles configuration.
If you want activation of users that login through social networks, we'll need to investigate why the link is returning a 404 error. Can you tell us what link is being created in the emails? With that, we can start to understand where things are going wrong.
Thanks,
-Melissa