Yeah. And Google has done the exact same thing. They're just fortunate that their business model is so much better that it can support their massively inflated head count. At least with Google it kind of makes sense in an evil way: hire every programmer who could be a serious competitor.
Difference being that Google recognizes that one of their strongest growth areas is capitalizing on innovative ideas. Gmail, Apps, Gtalk, Chrome... all innovations that build the company.
I'm not sure what facebook has done, or whether this should really be what the focus on, to be honest.
Perhaps also with some empire-building thrown in? Maybe everyone wants a promotion in recognition of their great work, so they need to hire new employees to work under them. I have seen hiring decisions made for that reason before.