I don't mean to be unpleasant (really!), but I don't understand what kind of answer you are expecting to get.
The only non-trivial answer one can give is also the most obvious: Facebook doesn't want to make free software.
Stated differently: why the assumption that Facebook is in the business of Free software? Or are you just expressing the desire for all companies to produce Free (in the Stallmanian sense) software? Or are you reacting to the implication that Facebook's software qualifies as free?
I only bring this up because these kinds of loaded questions have a way of devolving into flame-wars.