Those stats for "most online today" and "most online ever" are misleading, Jane, because they include all visits to the site, not just from registered members. And it appears that most of the visitors are not even living, breathing human beings, but "bots", scouting out all forum web sites, looking for vulnerabilities.
Probably the best guide to the intensity of forum activity is the Forum History that you can get to by clicking on the nearby
[More Stats] link. That shows how many posts there were each month throughout the forum's history. And you can even expand any month to see the number of posts and other activities each day.