It's realistically impossible to guage the amount of dotnetnuke sites out there, due to the large number of versions/variances of dotnetnuke itself as well as the fact that many run on intranets/extranets. In real terms there are more than 500,000 users with more than 4 million downloads to date, but even that misrepresents the figure as 1 download can equal many sites (note: in some cases 1 download can equal hundreds if not thousands of sites as many ISP's offer automatic installation tools to users i.e. press this button to install dotnetnuke). I think Shaun's recent post (http://www.dotnetnuke.com/Community/Blogs/tabid/825/EntryID/1655/Default.aspx) showing a small sample of the companies using dotnetnuke is a more interesting and applicable detail.

Cathal