Just my assumptions to this theme.

Renderosity is based on WWW pages. A web page is sent when there is a request to send one, there is no ongoing "connection" Renderosity could use. I click on a link, a web page is sent to me.
Renderosity uses cookies to see which user is which. Guests get cookies as well, so Renderosity's numbers most probably will include guests as well.

So, someone who get a Page delivered is online, and will be counted as online until a timeout is reached. If he requests another page, the timeout is reset.

So if the "Who's online" gives 1000 people, the other 2500 will be guests. If they followed a link leading here, they will be counted as "online Artist" until the timeout is reached. If that timeout is 5 minutes, 5 Hours or 5 days, I don't know. But you cannot see who is looking at a web site, not even if the web browser is still open. The only thing sites like Renderosity know is that there was one identifyable user or guest ordering a web site some time ago.

Does that help?

A ship in port is safe; but that is not what ships are built for.
Sail out to sea and do new things.
-"Amazing Grace" Hopper

Avatar image of me done by Chidori.