If I can build a Facebook client that covers the most important features within 3 months (at 2 evenings per week after work) then I'd bet that a company like Facebook (with hundreds of developers that are much better than I am) could invest the needed resources (maybe 3-4 developers per platform) to create a truely amazing native experience.
I'd even argue that it is not an option for anyone in their position to even consider going HTML5 just to reuse some code. That would be just the wrong place to save a few bucks for them, considering that they need to make a transition to mobile pretty soon and failing to do so would be an open invitation for competitors to come and basically kill them.
Additionally I'd like to add that I am not convinced that is even possible (or if so many times harder) to create a content rich application like Facebook with webviews wrapped in native code that feels as fluid and responsive as a full native app. I have tried this in the beginning with my Facebook client (app link: https://play.google.com/store/apps/details?id=com.flipster) and failed miserably before stopping this webview nonsense, and so did the Facebook developers.
On the scale of Facebook's resources, building well-functioning apps just isn't a hard problem. Not saying it's not hard, just saying it's not hard on the scale of Facebook's resources.
Additionally making the mobile app run smoothly on any device is just the bare minimum for what they should deliver. It's hard to not compare it against competition like the Android Google+ app, which absolutely crushes the Facebook app (I'm talking about the UI here, not about the social network itself), and that's where my sketicism comes from. Even if someone was able to deliver a WebView based app like Facebook, running smoothly on cheap devices, other competitors betting on native (like Google+) would still be in a different league.