The human mind can only follow all the social interactions of a group of around 150 people -- give or take. That number likely varies a bit from one person to the other and some historical figures, like Napolean, were successful in part because they had an amazing memory for personal details of people around them.
I was privy to the results of a good quality private study some years back when I was briefly the lead moderator for, iirc, the oldest community on a particular topic on the internet. The study was done internally by the very talented founder.
They found that 20 percent of members were active and regular users, 10 percent posted once or only occasionally, and the rest lurked.
Twenty percent is one fifth. If you multiply 150 by 5, you get 750. It has been my observation that when small online communities hit 750 members, they start having issues and needing to differentiate and/or formalize, which fits neatly with the above two piece of data.
The way humans deal with larger groups than that is formalization. Having formal customs and what not so you don't really need to personally know an individual to have a recipe for how to successfully interact with them is how people cope with having to interact with people they will never know well.
Hacker News has done a good job of formalizing certain things and that keeps the signal-to-noise ratio high here even though the sense of community it once had when it was smaller is no longer something it seems to have.
A subreddit can become a community, but you would need to keep it small. It might also help to make it private.
I hypothesize that you could foster more of a sense of community for HN by expanding the leader board to 150 names.