Facebook have given back plenty of open source to the dev community. Scribe (their federated logging server) and HipHop (their PHP compiler) come to mind immediately, but there may be others.
Their business tactics are ridiculously sleazy though. Aside from the google smear campaign fiasco last year, do you know that facebook does not allow AdSense advertising on it's 3rd party applications? They don't want FB ads performance to be compared with Google's (more profitable ads) on the same pages.
I somewhat agree. Frankly I'm no fan off Google, Facebook, or Apple. But pretending that one of them is more "open" then the other because they open source some software now and again is a falsehood: they are all for-profit corporations, with more in common with each other then what separates them, notions of culture aside.