According to Matthias Gelbmann of W3Techs, 25% of the sites it surveys are using WordPress. The milestone comes two years after reaching the 20% mark. Drupal and Joomla, two other popular open source content management systems combine for 4.9%, slightly less than 1/5th of WordPress. W3Techs counts both self-hosted WordPress and only WordPress.com sites that use domain mapping and have enough traffic to be in the top 10 million Alexa sites are counted leaving millions of WordPress.com sites uncounted. Only 1.25% of WordPress sites in the survey are hosted at WordPress.com. The survey also shows that WordPress is still the fastest growing CMS compare with Shopify as the second-fastest growing CMS.
You can view the screenshot showing WordPress’ rapid growth from year 2011 up to the present and for further information.