That seems foolish, if true.
Not developing for a completely unproven platform, where most users are served perfectly fine by the existing web app, isn't "foolish," it's a good use of resources.
This quote kind of reinforces my point. She only named two mobile operating systems. Facebook in a browser isn't a web-app, it's a website. Facebook in a mobile browser is coded specifically to use mobile human interaction standards, blurring the line between website and app - a web app. I still think he's talking WP8 and tablets right now. Notice how FB isn't making an OSX client as well, also a huge user base, and also a system that has browsers.
EDIT: After re-reading, I see what you're saying. I guess I don't see the relevance of having a specific W8 app unless it offers better/cleaner functionality than the browser. A lot of people do things in different tabs and windows and copy links to FB or vice versa.
With Windows being such as success in desktop PCs, a blurring of the two ideas is strategically important to Microsoft. If people starting thinking of tablets as PCs it benefits Microsoft. They are even moving towards having laptops be tablet-like with touch interfaces; and transformer tablet/laptops further blurs the line. From an app perspective this could mean either 1) that the reach of Windows will pressure app developers into supporting the platform or 2) that due to its closeness to traditional PCs, and most users being on traditional PCs, app developers will see their website (if they have one) as good enough. If I'm predicting, I think the former will happen.
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.
1) Microsoft is intentionally blurring the line between tablets and PCs. Usage patterns will be different between one extreme -- something like the surface -- and the other extreme -- a tower PC / monitor combo. But between those extremes there's a lot of middle ground; convertible tablets, laptops with touch screens, all-in-one desktops with touch screens. I don't think it's so easy to say that most people will fall into a traditional desktop usage pattern because a lot of those people won't be using a traditional desktop computer.
2) When you're dealing with a user base the size of Windows, even a small percentage of users using the Windows Store still adds up to a lot of people in absolute numbers; competitive with what the other tablet platforms will have.