WordPress is the most popular content management system (CMS) used for building websites.

Information from W3Techs shows that WordPress is used to power over 15% of the world’s one million largest websites and over 54% of all websites using a CMS.

You can view the statistics here: Usage of Content Management Systems for Websites

WordPress.org reports that 22 out of every 100 new active domains in the US are running WordPress, and the popularity of WordPress can be seen in Google Trends.

This detailed article from Smashing Magazine looks at WordPress, Drupal and Joomla and their use as a CMS: How WordPress Took The CMS Crown From Drupal And Joomla.

And you can also read up on some other related articles below – these articles use different information sources from different periods, but the overall picture is the same – WordPress is a very popular CMS used for building websites.
A Look at WordPress Market Share Numbers
WordPress Usage Statistics
CMS Usage Statistics
Joomla and WordPress by TechCrunch