Open facebook app instead safari / iOS - C#

I'm trying to call the oficial facebook App from my game instead open the safari browser. When the user click on my facebook button, I want to redirect him to my facebook page using the facebook oficial app.

I could using this:

 Application.OpenURL ("fb://page/xxxxx");

The trick is to use fb:// instead http://www.facebook.com... So far, so good.

But what if the user does not have the facebook application installed?

So, I tried this one:

  void OpenFacebookPage ()
  {
         WWW www = new WWW("fb://page/xxxxx");
         StartCoroutine(WaitForRequest(www));
  }
  
     IEnumerator WaitForRequest(WWW www)
     {
         yield return www;
 
         // check for errors
         if (www.error == null)
         {
             Debug.Log("Sucess!: " + www.text);
         } 
  else 
  {
             Debug.Log("WWW Error: "+ www.error + " Opening Safari...");
             //error. Open normal address
             Application.OpenURL ("http://www.facebook.com/xxxxxx");
   
         }    
     }

But it did not work as I expected - always return error. I guess it did not work because WWW not support the facebook protocol. In the Unity scripting reference:

Note: http://, https:// and file:// protocols are supported on iPhone. ftp:// protocol support is limited to anonymous downloads only. Other protocols are not supported.

So, I'm stucked here.. Anyone knows how to solve this?

Thank you!