Short answer:everything. 

They know more about you and anyone else because most of us spend so much time on those services. Every trip, photo, meal, purchase, comment, like, website visit, and click is logged and used to create a detailed profile of you. That profile is mainly used to do a better job of targeting ads to you. 

The number of data points that they have on an average user is in the tens of thousands. Just reading someone’s post is a data point. Liking a photo is a data point. 

The social media companies have image recognition and they know exactly what was in that image: a dog for example. Like enough dog photos and guess what, you’re profiled as a dog lover. Like a few photos with chocolate, now you also like chocolate. Over the course of 5-10 years of these likes, they know more about you than perhaps even your friends and family. They can see into your subconscious and use AI to determine things about you that you didn't even know about yourself.

If that data gets into the wrong hands, it can be used to manipulate individuals or masses of people.

Recommendation: Delete old posts you don’t need. Clear your advertising data periodically and use safe browsers that block trackers.