Denny Vrandečić, 07/03/2013 19:02:
http://simple.wikipedia.org/wiki/Special:Statistics
for active editors, and if I understand the definition at the latter correctly it means "one edit or action within the last 30 days".
Note that Special:ActiveUsers was removed by https://gerrit.wikimedia.org/r/#/c/55807/ but Chad confirmed that Special:Statistics is not affected (apart from the link), because the number is calculated in another way and cached (which also means it's not perfectly up to date).
Nemo