I've been reading more and more how social networking sites are being labeled as breeding grounds for infidelity. There is a great article about it on CNN HERE.
Although I think sites like Facebook can lead people to re-connect with old flames, I still believe that if someone is going to cheat, they will find a way - internet or not.
I don't judge anyone. I have been on both sides of the cheating spectrum and every person has a story. That said, it's hard for me to think that Facebook can be completely responsible for breaking up relationships. That seems a little too easy.