с помощью Facebook c # sdk:
string signedRequest = Request.Form["signed_request"];
var DecodedSignedRequest = FacebookSignedRequest.Parse(Facebook.Web.FacebookWebContext.Current.Settings.AppSecret, signedRequest);
dynamic SignedRequestData = DecodedSignedRequest.Data;
accessToken = (String)SignedRequestData.oauth_token;
var app = new FacebookWebClient();
var me = (IDictionary<string, object>)app.Get("me");
firstName = (string)me["name"];
U_Id = (string)me["id"];