To my knowledge, the users online module does not remove a user from being marked as online immediately after they leave.  So if you have 800 people visit in a 20 minute period (which may be longer), even if they only view one page then it will look like 800 people are online.

There are lots of other reasons for people to view a page on DotNetNuke.com then just using the forums, but that is probably where the most traffic is.  If you are like me and get notified of posts all day long, then just click through to answer a question, or even just click through to read the thread then you are keeping that number up.

Also keep in mind that the blogs are a good source of information, and that the online help for every module has links back to DotNetNuke.com

Are you asking why there are not more peole answering questions?  My theory on that is that lots of people would rather just let someone else answer.  If you are not approved to allow your posts go through right away then you might even answer it, only to have your answer wait in the queue while other people who answer after you have their posts go right through.