@BillRM,
I can't find the thread right now, but I remember asking questions like this a few years ago. Timberlandko's answers, as best I remember, were that about half the registered usernames never post anything. Of the remaining half, half submitted one post or more. Timber also gave the number of users with more than 10, 100, 1000, and 10,000 posts respectively. I believe about 80 posters had submitted more than 1000 posts at that time. Of course, these numbers are long-obsolete.
One figure that always interested me is what I call the size of A2K's core, C. If you list all A2K users, sorted by the number of posts they've written, the first C members on the list would have written half the total number of posts on A2K. I don't think I've seen a value for C yet, but I'd like to. (It's a nerd thing.) My wild, highly unscientific guess is that C would be around 500, give or take a factor of two.