Quantity isn't everything

That right there hits the nail on the head. There is a certain critical mass, an activity level that makes satisfy most discussion needs for most users. It's a tiny fraction of the total traffic of a place like Reddit or Twitter.
But if we have that, and keep the quality level up, we can succeed.

Success to me doesn't mean killing Reddit and Twitter. It means creating a place where smart people can come and find enough content and discussion that they don't need Reddit and Twitter.