I stand corrected. Site traffic is definitely a weak metric, and total users indeed probably isn't that interesting. I find the 200K active users/week and 3M

I stand corrected. Site traffic is definitely a weak metric, and total users indeed probably isn't that interesting. I find the 200K active users/week and 3M messages quite interesting. I admit, it would be more enlightening to know the distribution -- or at least something like the median number of messages per active user -- but that's a pretty good sense of the reach of Twitter.