Following previous posts on Google's disparity between Adwords and Analytics, it seems that Skype are getting in on the act: on the same screen this morning they claimed 13m and 22m onlines users.
This would bring a smile if it weren't for the fact that I've spent a lot of my professional career trying to explain and justify stats for online media.
It remains more of an art than a science, unfortunately.
This was the main reason we decided to introduce a heartbeat to all VidZapper player (and make it available as an API). Now you can measure video delivery to the nearest second...