"rewarding time spent on the platform"
Does a level system in a video game count as "rewarding"? What about achievements? What about a forum system with elements of achievements and likes (i.e. Discourse)? What about levels and achievements on Steam?

How would someone distinguish those cases from other cases?

The fediverse has likes (but no achievements, although it wouldn't be hard for someone to implement such a system, and I could see someone doing it for fun), is that rewarding someone?

That is the problem with vague demands as to what "platforms" should do.