From the article at least, it seems like there aren't a whole lot of PMs at Facebook.
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!)
From the article: 1/10 PM's to Engineers and 500 engineers = 50 PM's.
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.
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!)