The only "rules" in my marital relationship are, fidelity, honesty and respect.
After almost twenty six years of marriage, it has worked pretty good for us.
Internet cheating was in place long before FB, I remember back in the 90's all the chat rooms that were causing so many problems for friends of mine. The husband would catch the wife in the "Lonely and Married", and she might catch him in "My Wife Doesn't Understand Me and I Still Need Sex", (fictitous!).
If you are going to cheat, you don't need the internet, but it might make it more tempting or easier for those who have no real committment to their marriage or relationship.
My husband lives his life, and I live mine, then we live our "together" life. I would never presume to think he needed my permission to do a dang thing, and boy you bet that works vice versa.
Mine doesn't have a FB either, but if he did and he had ex's on it, I would just laugh and poke fun about how far he has improved in his taste of women. LMAO
The fidelity, honesty and respect takes care of the rest.