Facebook is a big partner with Apple. This year's iPhone was the first to have Facebook hooked straight into iOS, Apple's mobile software. Plus Zuckerberg is kind of a big deal, so it makes sense.
In the interview with Lauer, though, Zuckerberg downplayed the importance of Apple's iOS platform:
iPhone is a great platform. There are more people who use Facebook on Android — because Android is just — more people use it, at this point. And the thing that I think a lot of people don't think about is that there are actually more people in the world using Facebook on mobile Web, right, so not using the apps on iOS or Android, but actually just going to a browser on a phone. There are more people doing that than the iPhone and all of Android phones combined, right? So it's actually a pretty diverse ecosystem.