FB is here to stay until there's a new round of technology and FB doesn't move quickly enough to capitalise on it. I'm guessing that won't happen for at least another five years.
FB is basically AOL, which was basically CompuServe, which was a combination of DEC networking and BBS culture. (Kind of, if you squint hard and wave your hands around in the right way.)
There is always niche for this kind of all-in-one social/messaging/groups/etc. It's a very big and profitable niche, and someone always owns it. But the incumbents change over time, and there are disruptions whenever the underlying technology changes.
FB is likely a safe bet until something new appears. The obvious near-term move is 3D social/gaming, which is why FB bought Oculus. Longer terms that may not be enough, because it's not obvious head-mounted 3D VR is going to be pleasant and friction-free enough for everyday casual use.
My guess is the something new will have strong elements of AI and social and perhaps VR too - but I could be wrong about that.