Hacker News is successful because it is still yet relatively small. It will not scale.
Hacker News scaled from hundreds of users to hundreds of thousands of users and is still very high quality. Worse in some ways and better in others. Reddit scaled to tens of millions.
Just because Twitter is run by people that can't scale it doesn't mean it's not possible to scale it.
It's essentially an example of a benevolent dictator model.
Right now the people moderating are doing a great and largely unbiased job, but if you try to scale, you'll bring in new people, and gradually the quality of those moderators will vary with the larger group.
Reddit's sub-reddit moderator system is just one example of how to scale moderation to millions of users. If a moderator gets out of hand, reddit admins step in or users move to a new subreddit.
There are a virtually unlimited number of systems like this one could create and make work.