Why are so many people cheating on their spouses on facebook, etc. and don't consider it a form of cheating?

[Jasmine]

Look through my questions asked in my profile and you will see that there was facebook drama in my marriage. It was the fact that this woman said that she has been with my husband, which to this day still denies any involvement with her. Well I just decided to ask her why she is doing this and she says that it's because she has been with him for 2 years but claims that they dated since February 2011. That's not two years, that's three. To me that is just one way that I know she is lying. I also asked her that since she likes to send me pics of my husband and claim that she is with him, show me a current pic with the date taken. She responds with this: I am not wasting my time talking to you. He doesn't want to talk to you so let him go and move on. " My husband denies involvement and with that comment, it seems that she doesn't have any proof. At this point, I decided to walk away because it's he said, she said stuff and with her harassing me, I don't deserve that. She got into his facebook and now he no longer has one. If I choose to work things out, I think that one stipulation is that facebook is not allowed to be used. But what are you opinions on it? And can facebook really make or break a marriage? I mean, anyone can lie about any and everything on facebook. What are you thoughts?
Yes, I see that facebook is trouble. I use facebook for mostly business purposes but I even get tired of it. I deactivated my account. But this woman seems to be obsessed with him. Something had to happen and then because he cut her off or he lead her to believe that it was something and backed away, she just snapped. But it takes two to tango. I can see if she didn't know he was married but the fact that she says "Yes I know you are married to him but I am with him" means she is a slut.