Why would he post pics on FB? Because he's acting out and thinks he's in love w/this woman. He wants the world to know that he's found his soulmate and wants to share his happiness w/the world.
Then there is another thought...maybe he figures you or some of your family/friends will see the photos and tell you about them. Once you know, he figures you will finally get the message that the marriage is over and file for divorce.
My advice, block him on FB. Advise your family/friends that you do not want to hear about what he's doing. Don't say one word to him about the photos. Try to stay calm and start educating yourself on your rights if a divorce should take place, i.e., in other words, get your ducks in a row.
Don't ever think that they are going to live a wonderful life. Eventually the shine will wear off of this new and wonderful person and then the routine will set in...but it takes time. They have to have plenty of time to get use to each other and then the lies that they've told each other will surface the warts and cracks will begin to show and this all has to happen on it's own.
You, on the other hand, have to stay strong and start thinking about what you want and how you are going to move forward. Protect yourself and your family at all costs. Seek the advice of an attorney now and do not share that info w/your h. Do not agree to any financial divisions, etc. w/o speaking to an attorney to get the best advice possible. Your h will promise you the moon right now, but at the end of the day will not follow through on anything.
Sit quietly, the answers will reveal themselves when you least expect them to. The past is gone, the present is a gift and you need to focus on today, allow the future to reveal itself when it is ready.