i am executing the following code to logout of faceboook from my iphone app >>

if ([mFacebook isSessionValid]) {
    [mFacebook logout];
}

this code runs successfully and the delegate is called after this in which i am clearing the access token

- (void)fbDidLogout 
{
NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
[defaults removeObjectForKey:@"FBAccessTokenKey"];
[defaults removeObjectForKey:@"FBExpirationDateKey"];
[defaults synchronize];

}

but when i again login to facebook, it does not ask for username and password.

Can someone please suggest me what wrong am i doing??

Thanks in advance.