It's pretty erratic. You can see general trends by day and by week as graphs in the info tab, and there's a weekly writing exercise; you can find exactly when that is in your time zone by scrolling down a little here.
For the exercise, someone provides a prompt and everyone writes into it for ten minutes, then shares what we've got.
It can be; the system is "gamified" to reward desired actions. Depends a lot on whether you find a particular Stack that's focused on something you like thinking and writing about, and how that Stack's community behaves.
There's not so much difference, really. The "rules" in free-form games are shared assumptions rather than codified mechanics, but it's all different flavours of collaborative improvisational storytelling.