Today, seeing the "89,480 questions" I just wondered myself why "# Registered users" or "# Active users" (Active means here as the users that logged this month for example) does not appear on Super User main page?
Is there somewhere on Super User that already displays this information? If not, I just want to register my suggestion, I think this information would be useful to everyone, as the question number already is.
You can sort of infer this from
Since it's sorted by rep in that time interval, select a time interval and see how many pages it takes before you get to zero.
It's not exactly participation since you have to get an upvote (or rep some other way, like accepts or suggested edits) but it's close.
I've just on the way out, so can't be more verbose, but have a look at the full SE sites list and press "more stats" for the total number of users, and how many have moved to above 200rep in the last few weeks.
And, while you're on SE.com, have a look at the leagues - specifically the Reputation Change table on the right - as any users with a rep change were probably active in the selected time frame.
