I had a feeling that you would be getting a card from your in-laws. I had to chuckle because you had assumed that they had written you off. They probably posted your card late Friday or Saturday and thought you would get it on Monday. Because the mail moves slowly w/no Sunday delivery, it was delayed...but at least you got a card! Stinking thinking is not a good thing and it always brings you down.
No, I don't think your h is going off the rails. Plenty of people put photos out there like the ones you described. His friends may have friends that are now inviting your h to join them on their pages. Just because he's gotten a "few" questionable ones doesn't mean he actually knows them. But it will be interesting to see how all of this plays out. Eventually the novelty of FB will wear off, i.e., just as replay will.
Keep the focus on you!
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.