If you're basing it on forum users (which is a reasonable way to estimate a sites activity) you have to realise that as far as I can tell Stencyl forums are set to show active users from a long time ago. Here's one I just clicked on:
Yesterday at 04:21:46 pm
It appears their 'active' users expire after around 24 hours.
Try clicking on each active user and taking a look at when they were last active. On our forum, active users expire after only 20 minutes of activity.
Also worth noting is that our alexa traffic rank is 34,373 compared to Stencyl's 78,860 (lower is better)
http://www.alexa.com/search?q=scirra.com&r=site_siteinfo&p=bigtop
http://www.alexa.com/search?q=stencyl.com&r=site_screener&p=bigtop
A lot of these numbers are vanity statistics, and don't really matter in the grand scheme of things. What matters is that you are producing a good tool and have people that enjoy using it.