There is a worm out there right now that sets off automated attacks on forums such as this. I run a baseball forum that today showed 216 users online at one time. We checked the logs and discovered that many of those "users" were actually infected machines trying to attack our servers. We're immune to the attacks, and I assume the server running this forum is as well, so those attacking machines each show up as a user on the system.
These attacks use up bandwidth, but on servers that are updated properly, other than that they do nothing but make the user count look higher than it really is.