This person's phone has been tilting less often over the past decade - a trend correlated with, let's say, a more strained gait caused by weight gain. The algorithm markets them more sugary foods, because there's a higher likelihood they're effectively addicted to calories.

This other person is letting their battery run down more often - a trend correlated with undiagnosed depression. The algorithm ratchets up the fear and sadness because it infers these will be more effective motivators to buy products.

The best part is that this will probably all happen invisibly. GDP and human suffering creeping up slowly, with no human at the helm, and the world none the wiser.