I am sure Shawn knows exactly, but isnt this a little bit competition value information? Lets just say enough to not see your own ad on your own website all the time?
I thought I saw a post a month ago that there were around 3,000 sites that were validating. That figure stuck in my mind but don't quote me on it.
I don't think that Shawn cares, he posts the totals of the keyword tracker right on the login page. I saw that and wanted to know how many were on the co-op. 5k - 10k is pretty awesome.