如何获得朋友'；来自facebook API的用户名

我使用了以下代码，我得到了朋友的姓名、个人资料图片和id，但我需要朋友列表的用户名

protected void Page_Load(object sender, EventArgs e)
{
    FaceBookConnect.API_Key = "<App_id>";
    FaceBookConnect.API_Secret = "<App_Secret_Id>";
    if (!IsPostBack)
    {
        string code = Request.QueryString["code"];
        if (!string.IsNullOrEmpty(code))
        {
            string data = FaceBookConnect.Fetch(code, "me/friends");
            FaceBookData facebookData = new JavaScriptSerializer().Deserialize<FaceBookData>(data);
            foreach (FaceBookUser user in facebookData.Data)
            {
                user.PictureUrl = string.Format("https://graph.facebook.com/{0}/picture", user.Id);
                user.Email = string.Format("https://graph.facebook.com/{0}?fields=username", user.Id);
            }
            gvFriends.DataSource = facebookData.Data;
            gvFriends.DataBind();
        }
    }
}
protected void btnFetch_Click(object sender, EventArgs e)
{
    FaceBookConnect.Authorize("user_photos,friends_about_me,friends_photos", Request.Url.AbsoluteUri);
}

myJSON = string.Format("https://graph.facebook.com/{0}?fields=username", user.Id);