Which items on the list have engineers dedicated to rapid A/B testing running 24-7 to amp up the engagement numbers?

That implies that the A/B testing will lead to a new version that is substantially more engaging. Maybe so, but it seems that the most successful social media platforms arrived at their optimal version more or less immediately. For instance, I (naively?) doubt that much A/B testing went into designing HN. Yet, no other site holds as much of a grip on me.