I cannot authenticate Facebook user

I cannot authenticate Facebook user

what is the wrong on my code when use Facebook tab to contain game . I
test it out the facebook and it worked well and authenticated facebook
user and when i returned on facebook tab it wrked well. is the problem
with redirection or what ? . i hope if any one help i searched more but
nothing .
Facebook_login page
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
string host =
System.Configuration.ConfigurationManager.AppSettings["host"];
string facebookClientId =
System.Configuration.ConfigurationManager.AppSettings["FacebookClientId"];
string RedirectURL = "http://facebookapp.x.com";
if (Request.QueryString["redirectURL"] != null)
{
RedirectURL = Request.QueryString["redirectURL"].ToString();
Session["RedirectURL"] = RedirectURL;
}
Response.Redirect(@"https://graph.facebook.com/oauth/authorize?client_id="
+
facebookClientId + "&redirect_uri=http://" + host +
@"/FBcallback.aspx&scope=publish_stream,offline_access,publish_actions");
}
}