Just a sign of the times.
I was married before all the social networks got going, but there were opportunities to stray from my marriage the old fashioned way. One infatuation really excited me to the point that my wife told me to go to her and get her out of my system.
Yep, wife didn't need passwords to my Facebook account to spy on me, I told her everything about my life. That was part of our relationship, sharing - and her only concern about the other woman was how it would affect out relationship. Turns out the other woman was too much like me and I knew one of us would kill the other in a week.
Facebook is just the newest catalyst to end a marriage that was already ending.