It's the latter (using things like click through rates). I'm not sure if any one blog sums up their methods, but they write about this topic a lot. In one post

It's the latter (using things like click through rates). I'm not sure if any one blog sums up their methods, but they write about this topic a lot. In one post I read, they specifically mention that having a high quality camera correlates with their attractiveness ratings and so do different types of poses. So "attractiveness" might be better thought of as popularity, not beauty (though, that's the approximation).