I don't see Facebook disappearing, or people deleting their profiles en masse. What I do see is Facebook losing its status as the central location of people's online lives. There was a time where I left FB open on a tab much of the day, and ran the FB app on my phone - no longer. I know a lot of people who don't check FB more than a couple times a week.
Facebook is still good for messaging and chat, but its days are numbered as a place to go and waste time. That's what's Tumblr is for.