If he tries to point fingers again just tell him this :
Hold it.
You are changing the subject. Right NOW we are talking about facebook and your choices. You aren't a robot. I don't MAKE you do things.. YOU do.
If you want to discuss finances and how we manage that we can. But right now the subject is your choices to bring a SECOND affair into this home over face book.
Each time he tries to shift the subject to you accept that its worth discussing but confront him straight out :
a. Own your choices - you aren't a robot b. One subject per conversation
Each time he tries to change the subject and blame you for something shift it back and hit him again
a. Are you prepared to own your choice or are you going to hide behind blaming someone like an angry child? b. I will not accept blame for your choices. I will accept my choices, but I am not responsible for yours c. I am willing to discuss my financial choices and own them. Are you prepared to own your choices yes or no?
Just keep moving him back... don't let him get into finances.. Don't defend your finances, shift the conversation back and corner him until he agrees to take ownership or he runs away.. eitehr way he will learn something about himself.