At one point there was a lot of hype, regarding the possible use of Facebook for all or part of an intranet. From my point of view that quickly became a very bad idea. Maybe a new, small internet company, with internet savy executives could pull it off (maybe) but I wouldn't dream of trying to implement that approach where I work.
Aside from the questions of who owns your data? what happens if Facebook is down or slow? there is the considerable problem of bleed between work and personal life and bleed between the few useful things you can do on Facebook and the vast array of useless trash. The following youtube sums up my feelings exactly. I would suspect that most employees would not welcome this bleed either.
Not that Facebook is a complete waste of time, there are one or two people I can communicate most reliably with via a Facebook message, I have played a couple of very entertaining chess games via Facebook, but I have also ignored far too many hot potatoes and their like.