It's perplexing to me. I view HN as obviously being a social media, and (for me) the most time consuming one of them all.

Edit: Now that I think about it, HN is the only social media that I don't find it delivers enough value compared to my time spent on it (it does deliver value - it's just that it's a big time sink). I'm pretty happy with the time/value ratio of Facebook and (more recently) Twitter. It's probably because the ability to carefully curate what you see on FB and Twitter.