It's back up.

While they were down, they responded with a simple HTML page [1] and a 503 noticeably devoid of any interesting cache or CDN headers.

[1] http://pastebin.com/LaUX4Hp9

The Messenger app still works, but it looks like Facebook.com, Messenger.com, and the Facebook app are all broken at the moment.

I wonder how many 9's this cost them.