Who uses Mercurial

Mercurial is a free, distributed source control management tool. It is currently used by many projects such as...

Facebook

The social networking service Facebook chose Mercurial in 2014 due to its extensibility and the ability to make it work at scale.

https://www.facebook.com

Mozilla

Mozilla is an open source project that is currently developing the popular Firefox internet browser, the email client Thunderbird and the application suite SeaMonkey. Mozilla chose Mercurial in 2006.

https://www.mozilla.org

Nginx

The nginx web server is among one of the most popular and used over the world.

http://nginx.org/

and others...

Not just Mozilla and nginx use Mercurial. A lot of projects of various sizes have decided to use Mercurial for its efficiency and power. A longer list of projects which use Mercurial can be found in the wiki.