I think the way people use Facebook has the potential to be harmful to relationships. However, I do not think that the responsibility or blame should be on Facebook. Just because it is easy to message people, flirt, spy on your partner or cyber stalk them and get jealous, Facebook does not make a person do those things.
People harm their own relationships by behaving badly. With or without Facebook, someone who would do it on Facebook would probably do it off of Facebook eventually. I think people need to be responsible in their relationships on and off line.