> Once a social platform is around long enough, it becomes the thing that old people use (since its original users grow older with it)
I think this is true, but it's worth also considering the factor of how well a platform is able to keep its users segregated into their own echo chambers wherein they feel as though the platform as a whole meets their standard of what is hip and cool
Take Instagram for example. Ten years on from its inception, it's still very, very popular with 18-24 year olds isn't it? Simultaneously though, as I'm sure many Americans with a ~45 year old mother will be able to attest to, it somehow maintains very strong popularity with middle aged women too.
My idea as to how it maintains its general appeal while catering to these two disparate audiences is that its software mechanisms are carefully designed to keep both audiences separated from each other. Consequently both feel as though it's 'their' platform.
It seems that when a social media platform fails to build an intergenerational iron curtain like this, you end up with something like Facebook, where both generations gravitate away from the platform.
I admit I could be completely off the mark here though, and applying my personal experience more broadly than I should.