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.
People say HN is small and then cite Reddit or Facebook as examples of large forums. I would say that neither Reddit nor Facebook are communities or forums at all.
I would say that both Reddit and Facebook are platforms that host communities. Subreddits are communities and Facebook groups are communities and that's what you need to compare HN to if you want to gauge how big it is in comparison to other communities.
Comparing the 5 million visitors per month HN gets to the half billion members of Reddit is an apples to oranges comparison. I don't think it works.
As a platform, HN isn't very big. Reddit, Facebook and many other platforms have more traffic.
As a community or forum, I think HN is quite large. As far as I know, it's the largest and most influential (English language) tech forum on the planet.
Size does matter. The human mind only really computes a sense of community for a group of about 150 people. Above that size, you start having social dynamics like a big city, not like a tribe or village.
I generally define online communities in terms of "a group with one set of overlords." Platforms that have many subgroups with many different moderating teams strike me as platforms that host many communities. This is how Reddit works.
HN has one mod team. It has no subforums. So I think of it as a single community because it has some degree of cohesion provided by having one set of moderators (and a single space -- it doesn't really get broken up by topic, etc, the way some things do).
This is one of the reasons I feel HN is actually a very large community. Most platforms of this size or larger are more like Reddit and have been broken up into smaller subgroups.
Those subgroups are your communities. The fact that different people are in charge of different groups will foster different cultures, etc, even if the topic is nominally the same and the rules are nominally the same. They will be interpreted differently.
I've left a few other comments here already touching on some of this, chiefly these two:
Reddit as a whole is huge. I think of Reddit like a megalopolis, not a community.
I think some of the subreddits are communities. It would be sort of like a strong neighborhood association within a really big city.
The neighborhood or neighborhood association can have a strong sense of local community. The city as a whole probably cannot.
HN gets about 5 million unique visitors per month. It's the largest forum I consistently participate in. I'm not personally aware of any other "tech" forums that are larger.
For example, Lobste.rs is much smaller. And that's one of the names I hear the most.
According to this source, there are currently 58 subreddits with more than 5 million subscribers:
https://redditmetrics.com/top
Some of them are for STEM topics and maybe Data Is Beautiful is sort of like HN, but I'm not seeing anything obviously aimed at programmers the way HN is somewhat positioned. (Feel free to correct me if I'm wrong.)
Facebook is also huge. I want to say it's got like a billion users. But it's not a community of a billion people. It's a bunch of individuals with a curated list of "friends."
There are Facebook groups. That's where you would look for "communities" on Facebook. I have no data on them and I'm not going to look.
I would be curious if anyone else can even name any other online communities with at least 5 million members that aren't on either Reddit or Facebook.
From where I sit, HN looks quite large and looks to me to be possibly one of the largest tech communities. I welcome any data that suggests otherwise.