code: public String logout(Context context)
              throws MalformedURLException, IOException {
          Util.clearCookies(context);
          Bundle b = new Bundle();
          b.putString("method", "auth.expireSession");
          String response = request(b);
          setAccessToken(null);
          setAccessExpires(0);
          return response;
      }
docs: Invalidate the current user session by removing the access token in
  memory, clearing the browser cookie, and calling auth.expireSession
  through the API.
  
  Note that this method blocks waiting for a network response, so do not
  call it in a UI thread.
  
  @param context
  The Android context in which the logout should be called: it
  should be the same context in which the login occurred in
  order to clear any stored cookies
  @throws IOException
  @throws MalformedURLException
  @return JSON string representation of the auth.expireSession response
  ("true" if successful)