Social validation goes further than the site. HN for me is not only a filter for quality news but also for quality people. I'll gladly meet up IRL with a HN user I haven't met before, Reddit & other niche interest sites not so much. Also, "Are you on HN?" makes a great job interview question.
What HN brings in addition to that though is real social validation. If the HN crowd have voted up a story, I usually know its worth being curious about.