2007 Joomla User Survey Says: Who uses Joomla? - Joomla Expertise
We also asked a couple of questions about people's specific background with Joomla.
The Joomla experience of respondents is fairly evenly spread. One item of note is that Joomla is not yet two years old (September 2005), so the 16% of users that say they have been using it longer than two years are really saying they started with Joomla's predecessor, Mambo.
OK, so we'll assume a survey about Joomla has people that use it, but regardless, the result does show the popularity of the next few CMS's among respondents. Obviously the Joomla result is over-inflated as this is a self-selected sample :)