They do work. You can draw a straight line from “Engagement metrics” to a working business model that generates real revenue. There are several B2C products that derive their value from engagement. And it is frequently a leading indicator of how viable a product can be.

But like all metrics, one should not over optimize on any one metric - be it engagement, revenue, growth rate etc.

The biggest issue has been a lack of balance between engagement and usability which unfortunately has been dominated by engagement over usability.