If someone used HN to find interesting articles, then spends 90% of their time reading the articles and only comes to the comments briefly to see that other people think, then it would be fair to call HN a news aggregator site for that person.
But realistically, for most people, it's the opposite - 90% of time chatting with people in the comments, with the actual articles (or even just their titles) mostly just used as conversation starters, with the conversation often veering into wildly different threads that barely relate to the original topic. That's social media.