Off the top of my head:
-Total number of users
-Posts/Comments in a single day
-Active users in a single day (as in, those who posted or commented)
-Lurkers per day (people who "vote" but don't post/comment)
-Post listing on feeds (not sure how I'd figure this one out. I guess basically a klout rating for posts? Not sure this is a good idea. I'll think about it)
Right now I'm mostly curious how/at what speed hubski is growing, and what that growth looks like in terms of content and use of hubski. It will definitely be interesting to see what users become the "face" of Hubski, and if that occurs. I know that this is a little purist of me, but I think there's got to be a better way of doing this than the "popular" tool, since that runs into the same problems of just promoting what's already popular and snowballing.