Because myself and others have been frustrated by the lack of good
stats on the number of active editors on the English Wikipedia, I have
compiled some stats on the editing frequency on enwiki:
http://en.wikipedia.org/wiki/Wikipedia:Editing_frequency
I am going to forgo any extensive analysis for now. But I will say
that these trends mostly mirror trends seen elsewhere, with a peak in
early 2007 followed by a decline and then leveling out as we go
towards the present.
In September, 130,000 registered users and 525,000 anons made at least
one edit to an article. If you define "active editors" as those
making at least 20 article edits per month then 14000 registered users
and 6000 anons met that threshold in September.
-Robert Rohde