I don't work at Youtube, but different teams probably optimize for different things. Time spent is really, really hard to change directly, so they probably focu

I don't work at Youtube, but different teams probably optimize for different things. Time spent is really, really hard to change directly, so they probably focus on things like p(like), p(watch > X% of vid), p(watches another video). Generally speaking there's no answer in principle to the question "is the time spent actually valuable". For the vast majority of people, time spent on Youtube cuts into time spent doing basically the same thing somewhere else. We're not all researching cures for AIDS or new sources of green energy. Most people just want to sit around being entertained.