I have read the responses and have to say my head is reeling from the entire facebook subject--apparently it is actually from the devil. I don't know, I'm still trying to figure it out. So I can't help you from that angle.
I would definitely agree that, while this truly is disconcerting, you should not try to do something that could make it all much worse just for the benefit of "knowing the truth". I think we would all be shocked if we learned how much of the "truth" we know isn't actually or technically the truth.
Husbands, (all people, for that matter) have a lot of reasons for needing to hide things sometimes. I have only recently begun to understand just how afraid of me my husband is. We have been married for 20 awesome years and have an amazing relationship. But he has fears. I'm really not scary--he would even tell you that. But that doesn't mean he can't be afraid. It sounds like this is the only "hidden" thing going on between you, so I wouldn't consider the trust in your marriage to be "out the door". But if you sneak around on him to try to learn his secrets, it will certainly make the whole trust thing go down-hill.
Instead, I would go back to the things that you did in the beginning to make him hopelessly in love with you. Likely, you absolutely adored him. He was the strongest, smartest, sweetest, funniest,.... man you had ever met. Whatever you did that leaked out that this was what you believed was what made him want you and only you. If he believes that still, he will find it hard to be interested in anyone or anything (more likely in this case) else. Then the facebook thing may be moot.