Alot of those "visitors" are actually Search Engine "bots". They hit a website and start pulling keywords to optimize their searches. That's why when you go to google and type in kingdom of drakkar you will get about 300,000 related pages pertaining to that search.
It's good in a sense in that it will index more pages in search engines.
It's bad in a sense in that it will falsely inflate the number of "guests" we have online in our forums.
The majority that were on when you saw that number of 500 were yahoo's webcrawler spider search.