I am not administrator but user too.
This may means that Facebook don't want to give you real email of this user.
Facebook now try to not give anything after scandals.
May be user set in his/her facebook account to not give his/her email to anybody.
So there is no garantee that you get real email.
But you can try to play with the option 'Request an mail during registration'
(in administrative panel -> Components -> Slogin -> Options (in top-right corner) -> Basic Settings) -> Request an mail during registration.
try to turn it on or off ))) may be you get some results (but no garantee).
You can turn on 'Edit data before registration' (near 'Request an mail during registration') - may be user want to check his/her email by self.
You can play with 'Confirm account ownership' - may be you get luck ))
But when I registered at joomline.net or other sites with my Facebook login, I registered correctly.
I think if it were something from Facebook it would be a problem for everyone and it does not happen that way.
Email data is sent according to user profile permissions. If for some reason there is no email (the user is registered by phone, the user has set privacy options). We do not get email, but since Joomla requires it, we generate an email based on facebook data.
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