The obvious example is how Facebook tracks your physical location and nearby devices to determine what ads to serve you. That’s often why people think Facebook is listening to them and serving them ads for things they mentioned in casual conversation but haven’t searched for themselves. They didn’t search for it, but their friend did, and they know if you get an ad for that thing you’re more likely to click through based on potential word-of-mouth effects. They’re using your IRL social network and physical location as an extension of their ad algorithm.