Because the prison system (and the national conversation) is dominated by sadistic people who are fixated on punishment. Of course some prisoners would try to use Facebook or whatever to carry on their criminal enterprise, but I really think it would be much more effective in 99.9% of cases for represetnatives of the prison warden/parole board to monitor and comment on inmate's Facebook pages - prising them when they do well (eg completing an educational program or achieving some work goal) and questioning when they don't ('Getting another tattoo may help pass the time, but it won't help you on the outisde.')