Update as of 1:26 AM PST, June 19th: Facebook is up again.
On June 19th, 1:14 AM PST, Facebook is experiencing unscheduled downtime. This is the first global downtime that Facebook has suffered, especially for several minutes. Not only is the website and app down, but the API is as well.
Twitter and various news sources indicate that the downtime is global.
It's worth noting that Mark Zuckerberg was quoted in The Social Network as saying that Facebook would never crash:
To quote a randomly chosen tweet: "Facebook? More like Facepalm. Got 'em."