Joomla was released in 2005, and while at one time it was more popular than WordPress, it now clearly trails behind by a significant margin. It is however, still the second most popularly used Open Source CMS available. According to W3Techs
, it has a market share of 3.7% at the current time, although they include hosted solutions (including Shopify) in that number. In total, Joomla powers around 2.5 million websites.
Getting a site set up running Joomla is a little bit more involved than WordPress, and the administration interface can seem somewhat overwhelming when first faced with it. However it was built from the start as more of a flexible content management system, as opposed to WordPress, which started off as a blogging platform, and later took on the role of a CMS.