blank white screen after FB login via web app?

All you need to do is adding ” display:’touch’ ” and redirect_uri parameters to the login as :

FB.login(function(response) {
   if (response.authResponse) {
     console.log('Welcome!  Fetching your information.... ');
     FB.api('/me', function(response) {
      console.log('Good to see you, ' + response.name + '.');
       FB.logout(function(response) {
         console.log('Logged out.');
       });
     });
  } else {
    console.log('User cancelled login or did not fully authorize.');
  }
}, {scope: 'email,publish_stream' , redirect_uri: 'YOUR_REDIRECT_URL' , display : 'touch'});