Good q. All of it is still Objective C and C++; there's a few blockers to starting to move to Swift, including the relatively large amount of custom build + development tooling that we and FB have in place.
It's getting used more and more in our app--few recent examples are the "Promote Post" UI if you're a business account and want to promote a post from inside Instagram, the Saved Posts feature, and the comment moderation tools we now provide around comment filtering.
