Facebook wants to be the App Discovery channel. There is a technology side and a business side of apps. The technology side is about choosing platforms for which to build your app and user experience. Facebook do not want to be the ones picking the platform but rather support an experience for all. The business side is about App Discovery, how a brand and app developer can get their app to their users. This is where Facebook can help, by offering a channel where app developers can get their users to spread their apps to their friends through e.g. the New Feed.
I think there is a lot of interesting things in his views on this and we have been knowing that App Discovery is gonna be huge since me and Lars Erik went to AppNation last year. When there are a million apps out there, how are you going to make sure users find yours, and how are users gonna find the one for them? I think Facebook might be a great platform for that. I too believe a lot in serendipity (as can be seen in several of the projects I've done over the last years like Push!Photo), and like Bret Taylor says: "Social means serendipity.". When combining this with his technology statement that "Making a social app is making an app ubiquitously accessible.", then I think it is easy to see that HTML5 will play an important role in getting all of that together.