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.
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.