Is total sales volume good metric to compare the Ubuntu, Apple App store, Amazon app store, Facebook App Center, the Microsoft Metro app store, and Google Play with one another?

Is the consensus that Google Play has too many free apps and they need to move many more apps for the same $ volume? Should app stores be working to establish a more enterprise-y (Amazon and Microsoft) or sophisticated (Apple) culture so that customers expect to spend money?

How serious is the potential for one successful store to sell HTML/CSS/Javascript apps that start eating the other stores' lunches?

Is Facebook in good shape because others' apps won't run directly in its domain? Or is it more vulnerable because it has so many vendors working against it to make "sticky" apps that could pull them away from the Facebook experience altogether?