Having been on the team that issued postmortems before, I can tell you that we said as little as possible in as vague a way as possible while meeting our minimum legal requirements. Actual Facebook customers (i.e. those who pay money to Facebook) will get a slightly more detailed release. But the whole goal is to give as little information as possible while appearing to be open. As an engineer that makes me growl, but that's how it is in this litigous world -- don't want to give someone a reason to sue.