The userbase will never move because its where everything is and platform migration is a chore for those of us who know what we are doing so heaven knows how the legion of FB users would cope.

What Facebook will do is slowly die as its userbase very tragically does so IRL. Facebook was fun when we were young and it was just full of goofy wall addons and photos from a night out. But we got older and had kids. Grandparents joined to keep up on their grandkids.

Kids don't want to hang out where their parents do and this is why Facebook has plateaued. It cannot attract younger users whilst its existing base is their dad and grandma.

So as millennials age and return to the earth, so will Facebooks' userbase eventually dwindle until it is but a digital epitaph for future historians and sociologists to study.