There are about 2725 “active users” (which I define as having a rep ), that’s about 1340 more users within 6 months. In particular, many more first rate contributors have joined since last time. Overall, the senior users (:=tenured folks) include 4 very active Fields Medalists — Terry Tao, Tim Gowers, Richard Borcherds, Bill Thurston — and actually the user with the most gold badges is Tim Gowers. Among all users, about 18 of them have reached the 10k reps required for becoming a moderator (the first to 10k was Greg Kuperberg, the first to 20k was Pete L. Clark, the current user with most reps is Joel David Hamkins with 27.2k).
There have been nearly 11,000 questions asked (some closed quickly, but surely 10,000 at least were legitimate ones). That figure is definitely stunning! I’m not sure I have a favorite, not to mention very many superb answers.
A fantastic website then, congratulations to them.
Relatedly, MO’s cousin for undergraduates, math.stackexange.com (still in beta as I write) is fast becoming very useful too, at another level.