Instead of visiting the Mall of America and shopping at only what the mall will offer, using outdated infrastructure-- you can instead visit a variety of towns (“instances”) which each host their own cute boutiques (“communities”) specializing in a particular sort of item (whatever the topic of the community is). The instances can have their own rules that apply to its communities, but communities can also make their own rules, like a shop can. Skip the crowds and barely functional Supermall experience and find a better place to engage in discussion and get information.

I think he’s just making it clear that only those with many, many users matter to him, because individuals generate little value for him. He probably only counted the most popular subreddits as worth noticing.