I have a gut feel about a core difference between topic/forums and systems like Facebook/Twitter/Instagram:
On forums the discussion is almost always led by an OP. The person with the most vested interest to gather insight and move the conversation forward. Much like in a face to face group conversation. With more than 38% of posts, the OP on this post has ensured this topic continues to live.
The key on social networks like Facebook and Twitter is to incent a much wider participation from a larger more random community. The OP rarely is part of a viral discussion. So on those networks the algorithm is tuned to bump up topics that have a much flatter long tail of responses. They are short, pithy and meant to be controversial to gain momentum.
And the other type of topics/posts gather literally zero response. They are mean to signal and gather a passive viewership without any real discourse. The huge majority of posts are like this I think. Facebook is an archive of virtue signalling and bragging.
Have you ever seen a viral discussion on Twitter? Eventually the OP plugs a product or some SoundCloud link. It’s like a meme to do that.
If you want to build a competing social network you’ll need to adopt this interaction mode somehow. It requires an algorithm to put a discussion in front of people who would rarely ever even bother looking at something. And making sure that many respond so that the OP can just eat popcorn and watch. This is social media’s best (and of course worst) feature - getting people to talk who normally don’t say anything. I call this a “forced discourse.”