FacebookSignedRequest.Parse был перемещен в FacebookClient в v6.
var fb = new FacebookClient { AppId = "app_id", AppSecret = "app_secret" };
dynamic signedRequest = fb.ParseSignedRequest(Request.Params["signed_request"]);
или
var fb = new FacebookClient();
dynamic signedRequest = fb.ParseSignedRequest("app_secret", Request.Params["signed_request"]);