Really? Why? Who's to say that "social media" (at least in the sense of a unified platform with a real name) wasn't a fad?
Plenty of people are moving to a more fragmented internet life, with personal messages on Snapchat, photos on Pinterest, discussion on Reddit, etc.
What people seek in Facebook, they'll start finding in more directed services. Facebook was made for a world where smartphones and affordable or free fast mobile internet access was rare. A shrunk version of the regular web version can be enough for adults who are already used to Facebook, but teenagers and children will flock to user experiences that explore the advantages and idiosyncrasies of smartphones.
Maybe Facebook the company will own some of these new services and interface ideas, but Facebook the old timeline/friends/profile service will get obscured by the new, shiny, more intimate new services.
As a purely empirical, nonrepresentative sample, I can see that most of my friends on Facebook are fairly quiet nowadays and my own account is virtually dead. It's still quite useful to have a FB a/c in order to authenticate on other apps; maybe FB will transforms itself into the aforementioned meta-tool...? Until it's superseded by a more specialised one...
That said, its entirely possible that "social" is a fad, and certainly that internet social is way to wide a network than old fashion neighborhood social (invest in NextDoor I guess :-) but I'm still convinced that the value proposition of amplifying your ability to maintain a larger web of relationships is still a thing. Not many people are natural super connectors and Facebook takes more ordinary people a lot closer to that.