Indeed.Even if the visitor signs up, we need him/her to be active. We don't just want to increase the number of registered users, but we need more active users.
I think one thing that a lot of people take for granted is how much you can learn from helping others. Especially online as you often need to be very correct and clear as to how you explain something, which forces you to revise old concepts and often come to understand things of the past from different angles.
One also gets a lot of exposure to a variety of fields in math, regardless of weather you understand something or not, it's healthy to explore concepts that you haven't been formally exposed to before.
What i'm getting at here is... somehow popularize this fact as a learning tool to lovers of math.