Using a UIWebView has benefits other than just developer efficiency. If Facebook wanted to run A/B tests in a serious capacity, they needed the ability to make many changes to thier UI quickly. With the 2 week turnaround on app submissions, as well as the tendency for certain users to not update their apps, they could potentially be waiting years for the results of iterative A/B testing if they stuck with a purely native app.
Now that they've had the opportunity to fully A/B test their app, and they have a good sense of what users find valuable (based on this news story, the news feed and photos), it makes sense to switch to a more performant native app.
Now that they've had the opportunity to fully A/B test their app, and they have a good sense of what users find valuable (based on this news story, the news feed and photos), it makes sense to switch to a more performant native app.