On a site like this the lurkers always greatly outnumber the posters. Over the past 24 hours we've probably had around 8000 unique visitors, of which 234 have posted links or made comments:

  > (let posters (table)
      ((afn (n)
         (let i (item n) 
           (unless (> (days-since i!time) 1) 
             (assert (posters i!by))
             (self (- n 1)))))
        maxid*)
      (len posters))
  234

So 2.9% post and 97% lurk. The proportion of posters is actually higher than I would have predicted, based on what I remember about such numbers from Reddit.

What percentage of readers contribute by modding (is that the right word?) comments or submissions?

[/me restrains overwhelming urge to point out the fundamental errors in the design of this poll. :]

[/me nourishes secret hope that the poll fad will die out soon.]

So, let's see: If you comment, but almost never submit, I presume you are nonetheless a "poster". So, that's me.

I sometimes feel guilty for not contributing more. Then I remember that if I were trolling the interwebs for interesting content I would be spending even more time not working, and I get over the guilt.

I think it's good that people don't submit any old thing that comes to hand.

Presumably many lurkers don't have accounts thus won't vote in your poll...

I have users who would request a report with the names of all lurkers who have not registered.

(I tell them 6 months & $100K and they say never mind.)

I'd also bet there are plenty of people (like myself) who only comment and very rarely post links.

I lurk and once in 6 months I may make a comment. without clickpass, i wouldn't have made this post today

Its good this way, with more lurkers than posters. Why? Because very often lurkers don't have anything good to say, and those who have nothing good to say ought not to say anything.

Unfortunately, there are those who don't abide by that, and thus we get those ugly trolls.

I've gone back to lurking having been rather swiftly shouted down by a group of identikit hivemiders determined to prove the negative cliches of programmers as petty, rude and emotionally retarded.

Edit: ooooh, click your little down arrows, that'll show me... lol

Oh my goodness can I really vote up both answers? It seems that way.