[vl]

I think Joel misinterprets what happened in the first place:

It turns out that using ML to optimize for immediate engagement has two unintended side-effects: 1) it produces junkier content, 2) it decreases long-term retention. For obvious reasons, building a model to optimize for the long-term engagement is way harder and takes way more time.

While in the long run new model is more profitable (due to increased retention life-long engagement goes up), it decreases immediate engagement metrics. When this happens, major accounts start to call in and ask why now they are getting less for their dollar, thus this preemptive explanation by Mr Zuckerberg.