Fetch images asynchronous from url

Hi, I am making a game in which I need to integrate Facebook leader-board and facebook friends invitation like in subway surfer. I used official facebook sdk and got it done. But the approach I am following is not efficient.

My friend invite screen has its own UI so I could not use the default popup comes with facebook SDK. I fetch all the friends list from graph api It provides me the list of all IDs of users . But I needed to get name and images of friends to show in a scroll list . I am fetching the images from url asynchronously but whenever I try to fetch the image my scroll list hangs up . here is my code

 IEnumerator start ()
     {
       
         url ="http://profile.ak.fbcdn.net/hprofile-ak-prn2/t5/1116929_100003613987476_290892640_q.jpg";
         WWW www = new WWW(url);
         yield return www;
         
         var texture = www.texture;
         this.gameObject.GetComponent<UITexture>().mainTexture = texture;
         
         
         
     }

I am using coroutines, but I dont know why my scroll hang up on scrolling if these are asynchronous ? The code is been taken form here . I have checked Threading but www can not be used in threads. I just want to smooth my scrollview which I made by using NGUI sdk during image fetching.

Problem 1 - How can I make smooth scrolling by asynchronous image download simultaneously.

Problem 2 - I am loading each image into UITexture of NGUI, which I instantiate at run time. But if one have 200 or above friends then my app's memory reach at apex point and eventually app crashes. Is there any other way to load large number of images so that they take reasonable memory .

Please guys help me to smooth my scroll view and memory issue. Any help shall highly be appreciated. Thakks :)