User.IsOnline Problem

Hi,
    I want to check whether the user is online or not. i have done the following code. but the problem is it show some users are online even they are logged out. I want to know how to check they are logged out.

        string user = (Login1.FindControl("UserName") as TextBox).Text;
        MembershipUser GetUser = Membership.GetUser(user);
        if (GetUser.IsOnline)
        {
            Literal ll = (Login1.FindControl("FailureText") as Literal);
            ll.Text = "This User Already Logged in!";
        }
This
code returns true (GerUser.IsOnline) for all the UserName. That means
if i enter ...