And as much as I'd like being truly open as an ideal, it falls apart when you're dealing with competition (not cooperation) and money. At best you try to keep things open enough.
I don't see how Facebook's build needs to be kept secret. It's a purely internal process and while they might lose something by giving details they can also gain if someone suggests improvements. That said, there are plenty of tings they need to keep secret. EX: Letting anyone export FB's full social graph would be really stupid.
And as much as I'd like being truly open as an ideal, it falls apart when you're dealing with competition (not cooperation) and money. At best you try to keep things open enough.