> Social media are interactive technologies that facilitate the creation, sharing and aggregation of content (such as ideas, interests, and other forms of expression) amongst virtual communities and networks.
Forums satisfy all of these requirements. The key factor is not what kind of content users can post but that users can post, and more importantly that they post with the primary intention of interacting with other users. This covers Hacker News and other forums but excludes guest books and contact forms.
The key factor is that a third party has an algorithm that decides what you gets on your feed, based on the content. This is used to feed you ads or occasionally steer the election of the most powerful democracy.
> The key factor is that a third party has an algorithm that decides what you gets on your feed, based on the content.
You are describing Hacker News.