Also, how can we tell what our growth rate is and whether it's increasing or decreasing?
Keep an eye on the views / day in the right column. It's been hovering between 1100 and 1200. I'd like to see it trend upward.
Also, you can gauge how many people have participated by looking at the badges page. The Student, Teacher, and Supporter badges are probably the most important. They tell us to how many people have asked, answered, and voted, respectively.
We can see where we stand relative to the other sites at http://stackexchange.com/sites