With the latest sharp increase in forum traffic, I think we may play a game in relation to that. The title of the game tells it all. All you need to do now is to post the figure which you think would be the most number of people online at once in the forums. You can check this figure going onto the
forum index page and scrolling all the way down. And click on the "users online" button.
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So I'll start with the guess and my reasoning.
Well the pattern of the most online tends to be on ATAR day. The second most is on Oct-Nov when the exams are. And the most tends to be ~1.2 times the second most. However with the fast rise in traffic, where it tripled, or even quadrupled means that the 1.2 rule may not be very accurate. In fact, I have not seen the site experience that sharp of a rise in traffic when the forum statistics are checked. So I think the figure would be roughly double the most online of today (and perhaps more?) so my answer is:
8560 users online