Seems reasonable that at a company of 60k, with hundreds who specialize in PR, you do not want a random engineer making the choice himself to be the first to talk to the press by giving a PR conference on a random forum.
I still think he should be fired for this kind of communication though. One reason is, imagine Facebook didn't punish breaches of this type. Every other employee is going to be thinking "Cool, I could be in a Wired article" or whatever. All they have to do is give sensitive company information to reporters.
Either you take corporate confidentiality seriously or you don't. Posting details of a crisis in progress on your Reddit account is not taking corporate confidentiality seriously. If the Facebook corporation lightly punishes, scolds, or ignores this person then the corporation isn't taking confidentiality seriously either.
That's the PR team, clueless.
There will be plenty of time to blame someone, share technical lessons, fire a few departments, attempt to convince the public it won't happen again, and so on.