Yes, Windows 8 will ship on millions of desktops and laptops. And almost no one using those devices would prefer a full-screen Metro app to the website. This isn't like Instagram or something where a native app is the service; the vast majority of people using Facebook are going to be doing it through the browser.

So you're right, Windows 8 will sell a lot of copies for PC usage, and maybe a few people will find apps they want to use in the store, but primarily, those people are going to prefer browser apps to Metro apps, guaranteed - store apps offer no advantage, in this case.

Now, Windows 8 on tablets? If that takes off, I'm sure Facebook will start to target it. Until it does, though, I think we'd all rather them spend their resources on the platforms that they know are important to the vast majority of their users.