Ah, but herein lies the difference. Memes seem to universally promote freedom of communication, communication about freedom, and a way to react to the oppression of the status quo. In China, this is counter to the powers that be, but in America, this is fully aligned with the anti-regulation party that is sweeping through all branches of government. Memes play into their hands, and so Facebook would never be under existential threat.

That, or a public reaction to their product (ie, clickbait fatigue - Facebook gets really good at local optimization for the most engaging news posts but creates a news feed that convinces people that Facebook as a whole isn't worthwhile).

It basically needs to be serious enough to overwhelm the internal incentives of maximizing key performance indicators.

I think there's the global maximum vs local maxima argument in favor of doing something too. Why do people ultimately spend time on Facebook?

I would say they don't do so because of micro-optimized KPI targeting: they do so because they feel time on Facebook is valuable and makes them happy. If Facebook can't deliver increases in that, then optimizations only take them so far.

Yup. There's definitely some Goodhart's Law going on here as well. Micro-optimized KPI targeting is an inexact measure of feeling like ones time on Facebook is valuable.