Morning humbug, how are you today.
Emotionally I know you will be all over the place. You don't need to make any decisions about your future. It doesn't have to be a Deal breaker. For some it is and others work through it and come out the other side with their marriage / relationship intact. So please don't worry about that.
IME there is the cheaters script.
Deny
Delete
Minimise
Deny it happened, then when faced with evidence, they minimise. Only admitting to what they think they can get away with. At the same time they will delete all texts, emails etc.
Drip feeding is some thing else you may get.
Please ignore and block the woman from FB. yes a lot of women would want to be told that they have been cheated on and there's been many a debate on here regarding this, but not to be told in such a horrible way. She's told you, now ignore her.
Very often it is a good idea to get him to leave to give you some space. It also makes him realise what living apart from you would look like.
Yes he will say it's the first time he's cheated. Personally I would look for evidence to confirm that, but i know it's not for everyone.
Finally, and I'm sorry this is long but I know exactly how you are feeling! this is not your fault. He did not cheat because of something you did or didn't do. He cheated because he is an entitled twunt who thought he could get away with it.
Oh and carry on trusting your instincts, even if they sometimes give us an odd message that we can't quite decipher, they are very rarely wrong.