Current recommendations systems for FB, Google/YouTube have switched from models that try to increase immediate engagement to models that try to increase retent

Current recommendations systems for FB, Google/YouTube have switched from models that try to increase immediate engagement to models that try to increase retention because it turned out that optimizing for immediate engagement decreases retention. (Latter is obviously much harder ML task requiring Q-learning, longer data series, etc.) In other words companies choose to sacrifice immediate revenue in order to increase long-term (optimally life-time) revenue from each particular user. You can see it an example of early precursor of ML that shapes user behavior: learning what motivates user to return and then displaying it.