Logout with facebook login with asp.net application and clear all cookies

My application is in vb.net and integrated with facebbok. Facebook id is used as openid w开发者_开发问答ith my application. facebook user logged in easily with my application. now issue with the facebook login user logout time. when user logout from the application its redirect to login page. Again click on facebbok login button its not ask for reenter userid and password and direct login with my application. so what was the issue come? i clear all cookies and caches of application. i used simple logout button of asp:button. i not used javascript sdk for connect facebbok with my application. all code done in code behind.

I used this link to coonect facebbok with my application

http://osnapz.wordpress.com/2010/04/23/using-asp-net-with-facebooks-graph-api-and-oauth-2-0-authentication/