I just can tell you what my experience is. I was unable to do it, but what you have to consider on Android is that a many customers are going to use crappy cheap phones with a weak processor, so every layer of abstraction added is going to hurt the performance a lot.
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.
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.