TechCrunch reports that Facebook is hiring a team in Seattle to work on "desktop" software. If you think about it for five seconds, it's got to be a browser. Of course they're building one, just as clearly as Google was working on a browser before they launched Chrome.  

If you want to play at the level that Google and Facebook are, you have to be able to customize the user experience, beyond what you can do in HTML. That means you have to have a browser. 

Facebook probably has a camera in development too. And every other thing that can be simplified by assuming you're only communicating with Facebook. And maybe clicking on an occasional link to a news story or catalog outside Facebook. 

Update: I did a quick podcast to explain why Facebook is a force for vertical integration.