The main problem with native development is how you are not as flexible as your HTML counterpart. Even with the massive amount of capital that Facebook has, a full native app would always be behind.
Obviously, I don't work at Facebook so I don't know all the details, but I remember when Joe Hewitt built the native Facebook app, while it was awesome, it was lacking many features 4 months after it's release.
Since developing on iOS can take as much as 2 times longer than HTML development (+ the release cycle with Apple approvement system), it makes sense, in my mind, that they opted to have as much of HTML 5 code as possible.
Why isn't it as flexible? And a native app on a mobile device shouldn't try and replicate a full blown web app, it should have only the appropriate features that you would use on the go, but it should do these well and be snappy (read native).
Maybe for you, but the general public expect the same feature set. They can be presented in a different matter, but in the end they need to match. Look at the comment here, some people are complaining about lack of feature on the current app.
Obviously, I don't work at Facebook so I don't know all the details, but I remember when Joe Hewitt built the native Facebook app, while it was awesome, it was lacking many features 4 months after it's release.
Since developing on iOS can take as much as 2 times longer than HTML development (+ the release cycle with Apple approvement system), it makes sense, in my mind, that they opted to have as much of HTML 5 code as possible.
DISCLAIMER: My first language isn't english.