That's a tough thing to measure, unfortunately. One solution might be to conduct a self-evaluation. I'd like to do that, but I'm not sure what it would show us. (Let me know in the comments if you'd like be to organize it.) Another thing to look at is our Greatest HitsGreatest Hits. Ideally, we'd have a panel of outside experts look at our site and let us know if we are helping or hurting the cause of distributing correct information about the Bible and it's interpretation. My sense is that we are helping, but my bias is through the roof.
In the days of USENET, we called these users "lurkers". Personally, I think this is a valid use case. Obviously, we'd like to convert these people to regular contributors. In fact, this is probably the crux of the question. Real numbers on reader are hard to obtain. One measure is that over the life of the site, we've had 121 people sign up for our newsletter. But 24 people have unsubscribed. There are many reasons to unsubscribe and many reasons a dissatisfied reader might keep a subscription. Making a (potentially) wild guess, I'd say that about 80% of our readers stick around. If you have access to our anonymous feedback pageanonymous feedback page, you'll see that 70% of our feedback is positive.