I believe it was discussed early on when the article states McNamee believed that's when FB started to receive emotional cues from users. But I can see why readers might want a more thorough dissection of that point.
It was when the emotional state of users started to be revealed at scale. That allows to leverage targeted messaging to change users behavior by catering their emotional needs.
The obvious reason to me is that people started posting not because they genuinely had something to share, but to garner more "likes."
Anything that is measured will influence behaviors.