I haven't bothered to keep track of such things here, but some of those visitors could be "bots" -- search bots like GoogleBot, AltaVista's crawler, etc. -- these aren't malicious, btw, they're just automated programs that index the web nibbling up new content.
The most visitors (including bots) my primary website's forums (
http://WildTexas.com/talk) have ever hit was 572 in early 2005, and my site's far less trafficked/far less popular than GreenHybrid.com.
There *are* some bad bots, but that discussion would be outside the realm of this discussion unless the site owner's wanted to delve into that. My guess is a site like GreenHybrid has more live human visitors than bots, but that may vary throughout a day -- all that new content (every single message thread, for example) needs to get indexed, and bots are how it gets up there in lights on Google, etc.