I worked up some of the original numbers for Cloudkick, and while it doesn't look like it made it into the article, we did try to estimate the actual number of

I worked up some of the original numbers for Cloudkick, and while it doesn't look like it made it into the article, we did try to estimate the actual number of machines running on EC2 (we mostly stuck to US-East) at any one time. In short, based on the incrementing IDs we knew the total number of machines that had been launched. By looking at EC2 machines we've monitored at Cloudkick (at the time, a good portion of the well over 1M servers we had seen were on EC2) we were able to get an idea of what portion of all machines launched on EC2 are still running. I don't recall the actual numbers off the top of my head, and surely there is some selection bias in servers Cloudkick monitors, but we eventually worked up some estimated revenue numbers and they were right in there with the $500m-$750m (independent) estimates mentioned in the article, so at least we seemed to be in the ballpark.