People post chickens on Craigslist all the time, as for FB there is a way around it. For instance I sell fish, so on FB I'll post photos and say something like "I have too many fish! Send message if interested".
I suggest you say something along the lines of "chicks/chickens in need of new home FREE" and be sure you let them know they are meat birds. If you are concerned about it being illegal to sell cornish crosses, well it shouldn't be a problem if you are just giving them away?
Otherwise I suggest you embrace the homestead life style and learn to process chickens. If you've already raised these birds to a certain size you may as well get your money's worth and eat them. Ask around and you may be able to find someone who will process them for you. If you have any Amish or Mennonite folk in your area you could ask them?