The ones who you've gotten to try, are they pretty active posters on other forums?

I think SN attracts a certain type of forum participant. I don't think it's great for lurkers, because you don't get the positive feeling of getting zapped for content. It's also not good for doom scrollers because SN isn't giving you the constant dopamine hit. The posts on SN tend to be something you actually have to read (not glance at) in order to extract value from.

I think it's really good for people who actually want to engage, but even then-only for a certain set of subjects that current SN users are interested in.