After a week of silly headlines, people would grasp the reality that actually its zero impact on life moving forward. It could well not be replaced with any other online anything, but rather a simple return to talking to people you know by other routes just like, well, 2 years ago.
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).
People functioned perfectly well before Facebook and they will function perfectly well without it. Facebook is nothing more than a tool, one that I've never used and do not plan on using. As for the 800 million more people using their network, that number is way way way inflated. They might have that many registered users, but I doubt the active users is even a quarter of that.
After a week of silly headlines, people would grasp the reality that actually its zero impact on life moving forward. It could well not be replaced with any other online anything, but rather a simple return to talking to people you know by other routes just like, well, 2 years ago.