Usage statistics are collected whenever a user starts calibre. Every calibre installation has a unique ID, this ID remains unchanged by upgrades and even an uninstall/re-install. This ID is used to collect usage statistics. Only this ID is stored, no other identifying information is collected.
Active installs: 3,453,857 (those that have started calibre at least once in the last 60 days)
Note that installs are not users. A single user could run calibre on multiple computers. A single calibre install could be used by multiple people.
Installs by country (240 countries, top 30 shown):
% of unknown IP addresses: 10.2%%
Most popular versions (Top 20):
Operating systems calibre runs on