On a very slow day, we get 2 - 3 new members. On a heavy day, we get 10 - 12. I think the average is around 5 - 6 per day.
And yes, less than 10% of the actual membership will post in threads, but I think about 25% read only.
Additionally, I speculate that 65% of the readers of the site do not have a membership.
This is based on quick number-crunching in my head. I have all kinds of different numbers to look at, but haven't run precise calculations lately.