Works both ways
If a prisoner can post how (s)he is doing on facebook, what's to stop everyone else from posting what a lovely time they're having whilst not being in prison?
A couple of weeks of everyone pointing out how nice it is not to have bars on their windows, and that they can just pop down the pub when they feel like it (and if they want to use the phone they don't have to smuggle it into the house up their bum), and surely the most hardened of crims will start to feel a bit down about this facebook thing.