There's a rule similar to 80/20 which is 90-9-1. 90% are just observers, 9% react/respond (think upvote), and 1% contribute. I wouldn't be surprised if the HN/Reddit/etc. is close to this or some normalized and skewed version of it.
I'm not sure about HN, but I heard people say that the main editors/volunteers on Wikipedia that do the bulk of the work are somewhere in the 1000+ range (somewhere in the low four-digits).