Facebook isn't going anywhere for a long time. They'll retain the privacy-clueless fossils with FB. They'll keep the aspirational set with IG. Anything even remotely competitive or alternative they'll just throw cash at and snap up.
But their valuation depends on extrapolating today's growth and cash flow into the future.
So even a slight slowdown in their growth rate or small reduction in margins can have a magnified effect on the company's valuation.
Since expectations of higher future valuation drives high compensation for strong performers, any hint of this unraveling will result in brain drain.
It's almost certainly already begun.
Yes, my mom will still be on Facebook in 5 years. But I won't be there, my kids won't be there, and my friends won't be there.
Since the network effect works in reverse, the users who remain will spend fewer minutes per day on Facebook and Instagram than they do today.
And ARPU is dismal for the fossils who think Facebook is the internet.
eg. like having a myspace, or a .hotmail, or a @aol email account
Social networks have to be both useful and trendy in order to fully succeed....
messenger and whatsup are both very useful, FB itself is less, and it needs to be somewhat trendy in order to survive
Personally I find fb very useful to keep up with family and friends, but that might be not be the same for other people, especially very young folks