Maybe have a heart-to-heart with your old friend.
My guess is you don't know the future bride at all? That may be the biggest reason for the snub. If it's a traditional marriage, then her family is paying for the wedding, and *perhaps* it's a smaller intimate affair, and she is being really picky on who to invite to the wedding, i.e. people she is familiar with. Maybe she doesn't even realize how good of friends you and your friend were in the past. You look at the guest list and you only see casual acquaintances, but to your friend and his future bride, they may be more familiar with them than they are with you. The fact that you didn't know about the wedding until he told you on Facebook makes me think you two aren't that close anymore....did you even realize he was close to getting engaged?
So maybe he's not inviting you to the ceremony because of his bride, and she's got his balls in a vice. Or maybe he just doesn't feel you are that good of a friend anymore. Or maybe he didn't realize you would feel slighted not being invited to the wedding after all the years you guys have drifted away from each other. In any case, I still feel communication is the key, and based on what he tells you is the reason you aren't invited to the ceremony, you can decide whether or not you want to go.