It's very difficult to find a common metric that is reliable, unless I have user generated inputs for that. Currently, it's based on Google search volume. Mostly for the domain, but sometimes that is not possible, and for Photoshop I have to use "photoshop ui design" to get a good estimate for how popular it is among UI designers.
It seems that in Figma's case search traffic will be extremely unreliable given that you navigate to the website to use it, while you use a local app for Sketch and others.