Here's a simple one that you can try at home. This isn't one of those 'the facebook app listens to me' privacy breaches, it's more concrete and provable. Take a friend you haven't talked to in a couple of years, and start chatting with them over WhatsApp. After some days of this, go to your Facebook feed and watch how their posts now appear prominently, even though they didn't before you started chatting.

This is simple and harmless, but this same information has other uses that are not so harmless. They may not be being used right now, but they may start to be used in the future