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.

DISCLAIMER: My first language isn't english.