Facebook comes up a lot in these discussions and it's important to provide some context. Facebook's HTML5-first strategy pre-dated the launch of the original iPhone. In the first native mobile app they were simply rendering the mobile facebook site in a UIWebView. There are so many better ways to do HTML5 and HTML5/native hybrid apps these days. Check out the Basecamp app, for instance. It's every bit as good as Facebook's mobile app but it makes heavy use of HTML5.