"cambridge analytica" is big and public one. Facebook did some experiments too, selling to depressed teenagers or something...

They know "things you wish you knew", they know what is beneficial to you, but they will use this knowledge to manipulate you, they will use stuff you don't know against you to persuade you and me on scale, and later we will write "I wish I knew" but nobody will listen. It's better to show ad about new nosql in town, and make some money, if they know you will make a mistake of trying new framework instead of sticking with what you know and what works, they will show you ads for online courses.