While you could do such a thing for a service that requires a permanent connection, like FTP, IRC or a MUD, trying to do so for a website just shows not understanding the fundamentals of the service.

HTTP is a simple protocol which basically goes: make connection, do request, get response, break connection, although with HTTP/1.1, if you have a batch of requests, you don't have to create a new connection for all of them. But one doesn't stay connected, so a "who is online" feature is just as meaningful as the current temperature of the site.

Abigail