Logout an external login

Hi

Asp.net, MVC 5, C#

I am using external social logins like facebook, google etc to login to my application which currently works fine. But when I click on log off the app doesnt actually logs off from the social provider. For example if you login using facebook, click on logout and click login again it logins again with the facebook account entered before. I have the following code at log off:

   [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult LogOff()
        {
           AuthenticationManager.SignOut();
           return RedirectToAction("Index", "Home");
        }

Please let me know how can I log off both from my app as well as social login

THanks