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Unfortunately, it's not that easy. HTTP is stateless, and the user's web browser only connects to the server when the user loads a page from that web site. So, if a person does not explicitly click "Sign Out" when done, a user going offline can only be detected by an absence of HTTP requests by that user over a period of time. There is no simple way to tell the difference between a user who has gone offline and a user who is simply idle for some time.
Currently, a web session expires after 20 minutes (previously 10, I recently increased it after I noticed some people were timing out). If you are logged in but do not click on a link or load any page on the web site for longer than this time, you are automatically logged out. So this could be used as a method for determining who is online, although after leaving the website without signing out, the web site would still think you are online for another 20 minutes.