HN is a single unit. It doesn't have sub communities the way Reddit and Facebook do. So I think individual subreddits are a more similar basis of comparison than Reddit as a whole.

I personally mostly participate on Reddit in relatively small communities with, say, 20k subscribers or 80k subscribers or a lot less than that.

There are different mods for different Reddits. If you don't get along well with one Reddit, you can go elsewhere. There can be membership overlap but you aren't necessarily subject to the "I shot the sheriff" phenomenon where no amount of trying to behave and fit in is ever enough because one or more of the mods has it in for you.

I have been on the receiving end of that. Such experiences help shape my conceptualization of the problem space and how to define a "community unit."

LinkedIn seems to be akin to Facebook in how it works. Twitter is extremely different in how it gets organized. I don't think it compares at all to things like HN or Reddit.

I'm not really familiar with Pinterest or StackOverflow.