获取facebook用户名

我需要获得访问我的页面的人的用户名…我遵循以下说明:https://developers.facebook.com/docs/authentication/signed_request/

我写了下面的代码:

string _authorizationUrl = string.Format("https://www.facebook.com/dialog/oauth?client_id={0}&redirect_uri={1}", FB_APPID, Server.UrlEncode(FB_CANVAS_PAGE));
string _signedRequest = Request["signed_request"] as string;
string[] _valuesInRequest = _signedRequest.Split(new char[] {'.'}, 2);
string _stringToDecode = _valuesInRequest[1].Replace("-_","+/");
System.Text.UTF8Encoding encoding = new System.Text.UTF8Encoding();
System.Text.Decoder decoder = encoding.GetDecoder();
byte[] _byteToDecode = Convert.FromBase64String(_stringToDecode);
int charCount = decoder.GetCharCount(_byteToDecode, 0, _byteToDecode.Length);    
char[] _decodedChar = new char[charCount];
decoder.GetChars(_byteToDecode, 0, _byteToDecode.Length, _decodedChar, 0);                   
string result = new String(_decodedChar);

我登录了Facebook，我得到了这个结果:

{"algorithm":"HMAC-SHA256","issued_at":1316602539,"page":{"id":"XXXXXX","liked":false,"admin":false},"user":{"country":"it","locale":"it_IT","age":{"min":21}}}

没有关于用户名的信息。为什么?有人能帮帮我吗?

谢谢