Here's a link to the topic which described my initial problem.
In short: the problem is that in some cases facebook-graph-api doesn't return the email address of the user.
Other stackoverflow mates suggested to use his Facebook email if he has a user name (i.e. userName@facebook.com) which I've done. But what to do if the facebook user doesn't have "username" there too.
What would you recommend? To redirect him on the page asking his email address?