Sean has the membership/active user stats. The moderators can estimate, but Sean has the numbers. We don't collect age or other demographic information, which in principle would be useful for advertising sales, but given the nature of the site, I don't think we're going to start selling ads for non-photographic products based on the demographics of the site (and given the demographics of the site, what exactly would those products be? "APUG brought to you by...Viagra").
In general, my feeling from what I know from Sean and from what I've observed is that new members per month and active members per month have steadily increased over time. I don't know if it's easy for Sean to graph those figures, but they would be interesting.