Communities are built on trust and relationships, what happens at social platforms is that people with "large communities" are just creating followings the same way that celebrities always have, which are a weird modern corruption of the idea of a community (or if you want the academic term for it, a parasocial relationship).
What makes a difference to me are the insights you get here combined with thoughtful answers, as in rarely the lowest common denominator type of discussions.
For that there are all too many forgettable social media sites.
Hacker News is a very large and impersonal forum. You can have interesting conversations on a forum, but the degree to which they are a community is the degree to which you develop relationships with the other people. I wouldn't call /r/videos (or any other massive subreddit) a community either, and they have the same level of social intimacy.
Agreed. Aside from recognizing some familiar usernames[1], unless you know someone from IRL or via another social media platform, the posts all 'pseudo-anonymously' blend together. The standardized comment formats, fonts, and command bar next to each comment help make it so, even if you perform per-page custom styling.
[1] my personal list OTOMH is tctpateck(sp?), dragonwriter, doreenmichelle, dang(ok thats a gimme), sklabnik