Six apps, six embedded web browsers.
Facebook did flirt with HTML5 but ditched it in the end. So that's not really true.
It's really amusing to me when "engineers" start talking about the "scale" of the UI. It's a client. Thin vs. Fat aside, if it's that fat it's almost certainly a bloated mess of redundancy and what is called "overengineering" (which is actually underengineering--that is, a deficiency of the application of engineering and architecture principles to the design of the application).