Dustin Kirkland, which is the leader of the Ubuntu Product and Strategy team has announced that probably more than 1 billion users use Ubuntu, if we count everybody that uses Ubuntu, Ubuntu Touch, Ubuntu server or other versions of the operating system.
Unlike Google, Apple or Microsoft, the Ubuntu users do not need to have a live account, so Canonical cannot know the number for sure, only guess it, and by counting all the people that gets updated from the official repositories, the Ubuntu developers get only a rough estimate.
Dustin has added all the public information about the Ubuntu usage and got a number around 1 billion.
For example, Dell, HP, Lenovo and Asus release laptops pre-loaded with Ubuntu, IBM, Dell, HP, Cisco, Lenovo, Quanta provide Ubuntu servers and Bq and Meizu released some Ubuntu phones.