Drupal.org tracks download statistics for contributed modules. So one way to get a sense of how many Drupal 5 sites are out there in comparison to Drupal 6 sites is to look at the versions of popular modules that are getting downloaded. In particular, see CCK and Views.

No, that won't work. You need to know what version someone started on. That's the bigger problem in finding migration data, to find out when someone joined up.

If someone started at version 4 and they've kept current all the way through to version 7 then that would be the kind of stuff we're looking for.

After all, the point I was trying to make was that it is very hard for people to upgrade across major versions, so plenty of them are stuck with whatever was 'hot' when they first started out.

The knowledge required for migrating drupal to a higher level version is a lot more involved than for building the website in the first place.