Can we say ".IsOnline" property is not a good idea to find online users?

Hi all,
I have provided following code for test purposes to determine which user is online. I use membership provider. There is a sliding authantication where timeout is 60 minutes.
My first impression is that this is not successfull at all. When i logged in first time, then i can see myself as ONLINE user. But when I refresh page after 10 of 15 minutes wthout loosing authantication, i can not see myself there any more. Can we say ".IsOnline" property is not a good idea for this purpose?
I searched several articles and most of them advise to play with Session_Start and Session_End events of global.asax. But trick is, Session_End event is not firing 100% everytime ):
Here is my code. Thanks
Imports System.Web.Security Partial Class t Inherits System.Web.UI.Page Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load If IsPostBack Or Not IsPo ...