If you go to the top level (FatCogs Community Discussion Forums) and look toward the bottom of the page under "What's Going On?" you will find the site stats pasted in below (or whatever is current at that time). So "Active Users" (currently 9, 4 of whom are members) is different than "Active Members" (of which there are currently 146).
Awwh, yes. Surely it is based on how many folks here in the Fatcogs community are actively contributing and are visiting the site on a regular bases. I don't know the details as to how it is calculated but I am sure that Doug will chime in.
Active Members are any registered users who have logged into the site over the last 30 days. Usually in discussion forums, Active Member in the 15-20% of overall registered users in a forum are pretty normal participation numbers. This is a fairly niche topic forum, so the number is a little lower here.