I agree with everything you're saying, but what you're describing isn't a social network. It's a private, invite-only community and those have serious limitations when compared to an open social network.
IMHO, HN and Reddit are at their best when random people whose opinions you wouldn't ordinarily hear from decide to comment. For example, It's pretty common here on HN for open source authors to chime in on threads where their product is being talked about and answer questions. On Reddit, you'll occasionally get highly detailed comments from people with hands on knowledge of the topic being discussed. One that stands out in my mind right now is when an LA city planner commented on a thread about building high rent condos vs building low rent units. He laid out all of the city building codes around new buildings and why the finances don't work out for the low rent units. It was one of the best comments I've ever seen on the topic on Reddit and it would never have happened had the subreddit been closed off or invite only.
Of course, then the problem becomes how much crap you have to sift through to find that nugget. But, If you deleted all the crap as "shallow responses", would the lack of posts still attract those nuggets of great content? I think a high comment count and a high karma count actually attracts higher quality commenters because there's a perceived greater chance a lot of people will read it.
The only solution I can think of is to have a lot of moderators constantly auditing threads, marking the high quality comments so they stand out at the top and outright blocking or banning anything too egregious. Crowdsourcing doesn't work for this because it gets weaponized and/or abused.
Ultimately, what makes or breaks a social network rests entirely on the decisions the mods make about what kind of content they want on their site.