No. It isn't the inmates who are using Facebook, it's their family members. There could be a whole demographic category of "people with family in prison" which could be quite valuable to the (numerous) services around that market. Private companies manage commissaries, telephony, etc. and charge orders of magnitude above market rate. There's a lot of money in those services.
Why can't inmates purchase anything? It might be more difficult for them to express demand for merchandise, but all of my tertiary and quaternary research suggests markets thrive within inmate facilities.
Well, it's probably fair to say inmates haven't got much purchasing power for the things that would be advertised on Facebook. Yeah, non-zero, but hardly a large segment of the market Facebook's going to go chasing down....