Oh, they want you on the app, so they would specially not show you stuff from friends, so you have to scroll more. If you know that there is no updates - it is not engaging.
But if I pay for no ads, do they need me to engage? Their income is fixed to 14$/month no matter the amount of engagement. Actually, they save server costs if I engage less.
Stock holders, fixed :)
It is a pretty good metric of how the company is doing. I.e. if paying customers still spend a lot of time on the app, they would pay the next time. If the time decreases, there is a risk they would not prolong subscription.