M.P.
Are you sure his wife (step mother) didn't block you? If there is that contention between you and her, I wouldn't be surprised if she saw you on your dad's friend list and took it upon herself to get into his FB (people leave them wide open all the time) and block you?
If your relationship with your dad isn't a healthy one, it might just be best to cut your losses and move on. If you find yourself in a situation where you must talk to him, like your situation at Christmas, then talk to him casually, tell him how you have been, ask how he has been, and leave it very casual. Like you would an old acquaintance you haven't seen since high school. If he wants there to be more of a relationship, he will seek it out. If he doesn't, then at least you didn't get yourself so involved again only to be disappointed AGAIN.
If you haven't seen a therapist about this, I think other's recommendations of seeing on is not a bad idea. They can help you really talk things through and give you tools to help you cope with him in the future.
Good luck!