I've always said while HTML5 (or any web app) has its place it rarely beats a native app. In the past it was Apple mandating developers to write web apps for the iPhone only to eventually release a SDK for native apps on the iPhone. Looks like Zuckerburg agrees with the premise that native is better too.
Along that note aside from not being hosted are W8 HTML UI modern apps less performant than their XAML brethren? I've always cringed when WinDev pushed HTML UI's for W8 apps but have no practical experience to say that's a bad direction. It's my understanding the W8 apps produced by Microsoft so far are all HTML and they seem to do was well as any WPF app I've used on the desktop. Does anyone here have any practical experience with HTML vs. XAML in W8 or HTML5 vs. native app that they'd like to share here?
This will be an interesting next couple of years as we go from a principally homogeneous computing world for desktop apps to the chaos that is the devices world we're diving into. I don't see any clear answers forming yet and this decision by Zuckerburg doesn't help clarify anything other than to second guess HTML5 as an option.