I think a marriage is already in trouble before Facebook can be blamed.
This sort of reasoning could be applied to all sorts of sins people commit, that they must have had a problem otherwise they wouldn't have been tempted. It really is irrelevant. Catholics believe marriage is indissoluble, so whether or not a marriage was "already in trouble" is a moot point.
Before there was FB there was MySpace, cellphones, then chatrooms, newspapers, workplaces.
There are many acquaintances people would never likely see again without facebook or things like it.
The little friends box tends to show who pays attention to your profile.
All I can say is if I were married the only social networking allowed would be a joint account used with one spouse looking over the shoulder of the other. The internet is a serious enough threat to marriage that I think husband and wife should have tools to be able to see everything that's done online by their spouse.
Someone who is going to cheat will find a way.
I would imagine there are many adulteresses who don't intend to commit adultery but end up doing so. I don't really like the term "cheat" because it is used in the context of so-called "serial monogamy" way too often.
It's just the manner of them getting caught perhaps is different.
No, there is no doubt in my mind that facebook is the ideal tool for presenting temptations of this kind to people under the guise of being something innocent.
There are women who have added me on facebook I knew in high school, I am quite certain they never
would have contacted me otherwise. I'm not suggesting any bad motivations. And yes, the suggested friends list tends to show girls who had an interest in me, or in whom I had an interest. (and they knew it)