It's only a small "win" for privacy - given that Zuckerberg is on the other unencrypted end of anything "private" this might send, but I can see it as an important win for a few people...
Of course, perhaps this is part of a cooperative or NSLed collaboration with the NSA, "Hey Zuckerberg! We need you to tell us which of your users knows how to use PGP, then give us their entire social graph, the social graph of everybody who's ever uploaded a photo with them in it, the locations of those photos, and any Facebook-javascript-bugged webpages they've ever visited. Thanks."