I think this is basically it. The problem is that any automated discovery mechanism is quickly ruined by automated spam/ads created to exploit it. So the most

I think this is basically it. The problem is that any automated discovery mechanism is quickly ruined by automated spam/ads created to exploit it. So the most reliable discovery mechanisms are still small communities of real people. But because they're good - they attract more and more people until the quality reverts to the mean, and it's mostly just junk/spam/ads again. Social media companies are extremely prone to cancer - if you will. A large part of long they survive is in how good they are at fighting off the junk created by their own users. In a way - the internet (or at least the DNS system) is just the top level social media company that made absolutely no attempt to fight the cancer. It's value died a long time ago, and it's mostly a wasteland of junk now.