We use BuiltWith to measure WooCommerce growth and compare to our competition. They just updated their entire database which includes 332 million domains, and WooCommerce just passed a million websites (up from 900k). The new number is 1,111,882 sites running WooCommerce which is close to WordPress.org usage stats of 1+ million sites with the plugin installed.
This is pretty amazing for an eCommerce platform which is just over 4 years old, but shows the powers of open source and WordPress.