It could be done and I had considered it, however, activity is very expensive to calculate in bulk, as you will realize if you are the first to load http://zelaron.com/forum/zelstat.php?num=100 at midnight GMT. (Results are cached for future page loads, so you have to be the first to get the long load time). Keep in mind that while the server is calculating all those activity figures, other page loads are stalled. Of course, that page actually calculates the activity of every user that has visited in the past 28 days... in any case, I have no GD experience, so you'd need to enlist Chruser's help on this.