Awhile ago they brought on Brandon Walkin (http://www.brandonwalkin.com/blog/) who is, arguably, one of the most detailed-oriented user interface designers on the planet. He's also a very talented Cocoa developer and apps that he's worked on have won Apple Design Awards.
Next, they acquired Sofa, another Apple Design Award-winning design & software firm who have created some of the best-looking apps for OS X.
Now they nabbed Push Pop Press, founded by Mike Matas and Kimon Tsinteris, who were both instrumental to the launch of the iPhone on the software side.
Facebook is building an army of UI designers/developers who 1) used to work at Apple, or 2) have built apps Apple has deemed best-in-class.
I'd be really surprised if Facebook isn't working on something big to go up against Apple and Google. There were rumors of a Facebook phone, perhaps Facebook is building their own hardware and need a world-class design team to craft its OS? I can't see Facebook sticking all this amazing talent on a redesign of Photos or Chat, or updating icons in Facebook's iPhone app.