The number is growing and constantly changing.
There are many sites that aren't in use too. Only bots crawl them.
My estimate right now: About 1/5 (20%) of sites being visited on the internet use WordPress.
In 2011, it was determined that 20-25% of all new websites were built on WordPress.
CMS market share:
WordPress has over 53%
Joomla has over 9%
Drupal has 6%
Others make up the remaining 30%
Joost de Valk of Yoast.com had a nice info-graphic made recently that will shed some light on this and more.
He may even want to answer this more thoroughly. 🙂
Infographic and more here:
http://yoast.com/wordpres
