The PM/engineer divide feels uncomfortable to me, but I have limited experience with these roles, so it's likely simply for lack of knowledge. I hope someone can clarify this for me.
If an engineer has an idea, they seem to be more encouraged to just find some people to work with, build it, and then roll it out to a very small section of Facebook's users and see how they react to it. Good ideas might then be taken further, but bad ones (Or I guess, ones people just don't care about) won't be.
(I don't work there though, so I have no way of knowing if that's right or not, sorry!)
That's not an entirely small amount for Facebook since they don't have a zillion different products and features that need to be owned, like a company like Microsoft or Google.