I love the term "nonsense aggregator" xD

Usenet's also a good comparison, and yeah, not social media.

Definitely agree on K/Mbin straddling the line because of its microblogging feature.

I think that Lemmy and Reddit are 100% social media.

Common/Wiki definition:

Social media are interactive technologies that facilitate the creation, sharing and aggregation of content, ideas, interests, and other forms of expression through virtual communities and networks.

Content aggregators aren't discluded. Especially in this case where original content can and does exist.

The biggest difference, I believe, as to why Lemmy is social media and a typical forum is not, is the sorting. In a forum, the discussion is chronological as in a conversation. Here, more likes gets you more noticed. In content AND in discussion. Thus there is incentive. Whether you care about likes or not, it exist and so does incentive for social relevancy. It drives what you see.

Next becomes use case. You CAN sort the comments chronologically, but nobody does that. You CAN just read and never post, but people also do that on Instagram. Maybe you don't care about likes and aren't trying to get them. But they exist, and other users do care. If I didn't care about Facebook likes, it's still social media.

Whether you like it or not, everything is socially manipulated on this site.

Maybe you don't feel the negative effects that are typically associated with social media, and that's great. But some people here do and can get angry/upset/defensive about being down voted. Either way, those effects are not a part of the definition, although the connotation does exist. And the same could be said about any social media. Some people are more headstrong and less effected. This site is not nearly as predatory as the big ones and (depending on your communities) don't always have the intent to drive your emotional response. But those communities and users do exist.

I only have Instagram installed because there's a few people who send me (usually political) clips so we can chat about them when we hang out or text. I'm not following anyone I know. I have added a few of the creators. I've never once liked or reposted anything. So can I now say Instagram isn't social media?

Perhaps subcategories could be created, but that's besides the point. This site absolutely fits at least that one definition, which removes all connotation and defensive argument that can be had.

OP is here interacting with a network of users sharing ideas that are being sorted by popularity, then viewing other posts sorted by popularity. This is socially driven media.