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:
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.