Welcome Madi! If you haven't read it already, check the About page and the Hubski Primer, they are a good place to start. The backbone of the site is similar to Hacker News but with many features added to improve the sense of community. Perhaps it's also interesting to compare and contrast.
In theory, any user can make any kind of contribution they want. In practice, the mechanics of the site are such that interesting links, texts or images by users that have more than a few followers get most of the interactions and attention. The site revolves around sharing the things you read on the Web with the people that follow you on Hubski. Those people then share your post with the people who follow them, who might also share it with whomever follows them, etcetera. Both posts and comments work like this; shared posts appear in your feed, shared comments in your chatter.
This post of mine from a long time ago might be interesting, it's one in a string of meta-discussions about hubski. That post and the one linked to have a lot of ideas and thoughts about how online communities should be organised. No shortage of research material, that's for sure. :)
While I can't put a number to it, I'm pretty sure that the actively-engaged group of people here is no more than a few dozen. I wouldn't be surprised if there are a lot of people who just read hubski without interacting, having articles and comments all neatly on one page lends itself to lurking quite well (as opposed to message boards, for example).