For many years, YouTube's engagement metric of choice was "hours of video watched a day". Their goal was to stream one billion hours of video a day on YouTube.
This resulted in a lot of problems, as explained by the Reply All podcast:
In 2016, YouTube hit their goal. They blew past a billion hours a day. But the problem is they did it at least in part by letting all these sort of fringe voices flourish on their site.
This January, YouTube tried to fix some of this -- they tweaked the algorithm so that it stopped recommending flat earth videos or anti-vaxxer videos.
But it’s much harder for them to regulate this huge category of alt-right provocateurs that they elevated during their pursuit of a billion hours. Because when those people break the rules, or tiptoe around them -- they do so knowing that they have power. Because their whole brand is that what they’re saying is dangerous, and the establishment wants to shut them up. And so any action that YouTube takes, risks coming off as censorship.
The lesson is that chasing engagement metrics is dangerous. If you're not careful, you'll end up with people spending billions of hours becoming increasingly radicalized and misinformed.