Back in mid-2013, I wrote a post called, "The Bootstrap Boom is Just Getting Started". Looking at the statistics, it was clear that Bootstrap was on its way to becoming very popular indeed.
Three years later, and its obvious that even those optimistic projections underestimated just how popular Bootstrap would become. Not only has Bootstrap grown since 2013. According to a range of statistics, its usage has actually grown by between 800% to 1,000%
The image below shows Bootstrap's market share in 2013, according to BuiltWith.com. At that time, Bootstrap ran 1.6% of Quantcast's top million sites.
The image below shows Bootstrap's market share in 2016. These numbers have grown enormously. Bootstrap's share of the top million sites has grown nearly 900%, so that 15.7% of the top sites now use Bootstrap.
Overall, BuiltWith records over 7 million websites using Bootstrap. Compare that to 170,000 for Zurb Foundation and 400,000 for 960.gs.
Wappalyzer shows Bootstrap with a 57% share amongst web frameworks (yes, not all of these are CSS frameworks).
How did Bootstrap become so popular?
A large portion of Bootstrap's growth has come thanks to the support of CMS's and frameworks.
Bootstrap is baked into the Laravel 5 and Joomla 3 core and so it is the obvious choice on those platforms, but it also dominates when it is not the default choice.
The most popular Drupal 8 theme is Bootstrap.
In the WordPress world, I looked in January 2013 and only found 8 WordPress themes using Bootstrap and several of those weren't on WordPress.org.
In mid-2013, there were 30 themes on WordPress.org that supported Bootstrap.
Now in 2016, there are 250 Bootstrap themes on WordPress.org! ThemeForest has another 2,500 Bootstrap themes for WordPress!