WooCommerce has had explosive growth. That's not new to anyone. But you may not realize just how much WooCommerce has grown compared to the eCommerce space in general.
The WooThemes folks wanted to know who was using their product and how. In this summary post by Joel Bronkowski, he describes how they went about measuring. They used BuiltWith data to get a list of sites that use WooCommerce and then went digging. They were even quite conservative in my opinion, not counting, for instance, websites that may have WooCommerce installed but not really running, or are only using it as a catalogue.
The research was thorough and quite manual. What intrigues me is how there really isn't a market leader in eCommerce. In other words, it is still a very ripe space. There is no WordPress-style dominant platform in eCommerce. But WooCommerce has momentum and WordPress in its corner. I think you can guess where I think it's going.