By allowing us to write once and ship across multiple platforms, HTML5 has historically allowed us to keep the Facebook mobile experience current and widely available, and has been instrumental in getting us to where we are today. We chose to use HTML5 because not only did it let us leverage much of the same code for iOS, Android, and the mobile web, but it also allowed us to iterate on experiences quickly by launching and testing new features without having to release new versions of our apps.
So while utilizing web technology has allowed us to support more than 500 million people using Facebook on more than 7000 supported devices, we realized that when it comes to platforms like iOS, people expect a fast, reliable experience and our iOS app was falling short. Now that our mobile services had breadth, we wanted depth. So, we rewrote Facebook for iOS from the ground up (I really did open up Xcode and click “New Project”) with a focus on quality and leveraging the advances that have been made in iOS development.
The whole thing is an interesting read, even for the non-techie.