2007 Joomla User Survey Says: Who uses Joomla? - Building Joomla Websites
Some of the more interesting results from the survey came from questions about the type of sites that people have been building with Joomla. We asked a number of questions on this topic.
In the first of two questions, we see that the overwhelming number of people are involved in more than one site. This seems to confirm some of our earlier findings that Joomla is heavily used by web designers and developers creating sites for their clients.
This follow up question shows that most of the sites that respondents are involved in managing are powered by Joomla.
Among our respondents, Joomla is still most used for small traffic sites. There were a few sites getting over 5,000 visitors a day, which is notable and impressive. That's a lot of traffic for a site, roughly equivalent to an Alexa rank of under 20,000 (I think). About 100 sites fell into this category (if you are one of them, please contact us so we can provide some links!).