Facebook Graph API won’t return email address

The Marcus’ answer leads me to the real problem I was facing.

Debugging with the Facebook Explorer tool I was able to figure out that I should set the email scope at the FB.api() method after a successful login.

FB.login(function(response) {
    if (response.status === 'connected'){
        FB.api('/me?fields=id,email,name', function(data) {
        console.log( data.email ) // it will not be null ;)
    })
}, {scope: 'email'});

It is not quite clear at the Quickstart guide and I hope it helps someone else, too.