It depends on how you define engagement, I suppose. I've never heard people discuss time spent as an engagement metric, which is (IME) usually actions taken by

It depends on how you define engagement, I suppose. I've never heard people discuss time spent as an engagement metric, which is (IME) usually actions taken by a user that the business wants to measure. Engagement just measures how engaged your users are. Time spent measures that. These are all proxies for value, because it is very hard to measure value received.