I've been doing some statistical analysis of Drupal sites in preparation for a new site I'm launching. The results are very interesting. Here are some version statistics, based on 1,370 Drupal sites with accessible changelog.txt files.
In case you're wondering about the freshness of this data, the majority of sites surveyed were launched in the last 12 months, including sites published as recently as 2 weeks ago. The version numbers recorded reflect the Drupal version the site is running today.
The URLs were collected from DrupalSites.net, the largest public database of Drupal sites. Their list is constantly updated with user-submitted sites (with more than 1000 new sites added in the last year), and in theory, should be statistically neutral, as the motivational factor for publicizing a site is not dependent on which version of Drupal you use.
I have verified these results against some creative Google searches, and the numbers correlate very well. I have high confidence in the accuracy of these numbers as an overall representation of the Drupal install base.
A short analysis: Very few people are using Drupal 6 (in production) yet. In fact, Drupal 4.x installs outnumber Drupal 6.x almost 6 to 1.
The Drupal 4.x numbers are especially interesting, because 4.7 support and security updates were discontinued in January. That means 21.6% of live Drupal sites are running an unsupported version of Drupal! In fact, I even found five sites running Drupal 4.5.x, but the number was too small to graph.
Update: Some of you have expressed concern that the sites polled are all "self promoted" sites. This isn't strictly true, anyone can add a site to DrupalSites.net, it doesn't have to be the site owner. Either way, self promotion isn't something specific to Drupal 5 users, and there is no reason why a Drupal 6 user would be any less likely to add their site to the list at DrupalSites.net.
Self-promoting sites or not, the indisputable fact is that of the 1370 sites surveyed, 96% of them have chosen not to upgrade to Drupal 6 yet (note: I'm not implying they will never upgrade).
There are plenty of real reasons why users aren't upgrading or launching new production sites on Drupal 6 yet (It's still kind of new, modules aren't ready, etc), but that's not what this article is about. I'm strictly interested in the numbers, you can draw your own conclusions as to why they are the way they are.
Stay tuned for more statistics and graphs.