There is no official Limit afaik, but Facebook will definitely limit your app if many users mark your entries as spam or block the app. Which will most likely happen if you post something on the walls of the user´s friends.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.