I like Cory Doctorow's @doctorow analysis of how social media giants fall.
In short: in the process of *enshittification* (get on it, New Palgrave), network externalities allow the platforms to capture almost all of the surplus and sell it to advertisers etc, leaving users nearly indifferent about leaving the platform.
When a shock hits the system, the switching cascade can be rapid because no one has a good reason to stay around anyway.