You are conflating facebook and the internet. That traffic would NOT return to the mail service. Remember, there was email before facebook messaging. The phone services still have major SMS traffic and push notification systems like Apple's are used to announce FB activity.

I'm already finding my self moving away from FB towards dedicated services like Path, Instagram, and GMail. I'm starting to find myself using FB about once a week.

Digital sharing will never go away. But I wouldn't be surprised to see Facebook one day replaced by more interesting alternative(s).