When people sign up to protect their website against spam with Mollom, they are asked to categorize each of their sites. So far, almost 2,000 Drupal sites have been categorized. The available categories are: a company website (22%), a site built for a customer (7%), a non-profit website (27%) or a personal website (44%).
It is only one data point and a relatively small sample so I don't know if it is safe to generalize, but I figured it was an interesting nugget that could help us understand Drupal's install base.
— Dries Buytaert