- facebook, digg/reddit and twitter kill independent forums. They are 'good enough' mostly for friction reasons that people slowly move there. I think the twitter part is unappreciated - in the past when you had one simple issue, you had to register to communicate it. Now people are likely to tag the profile on twitter with their problem. Sometimes these people remained, now that user acquisition channel is lost.
- they suck at creating communities (I think intentionally), so people don't contribute as much.
- discord is a chat app with flawless ui. As a chat app it's great at building communities due to frequent interactions and instant feedback. This means most active people do everything there, same people who would have 1000+ posts on old style forums.
The tl;dr is that forums are best at preserving information for outside access, but are inferior in making new people join and contribute, which means they are never going to proliferate again. The old optimal balance of chat vs forum is never going to return.