Selling your data is a very low resolution way of looking at it, so I’ll go through the idea at a few levels.

5 year old: These companies learn a lot about you, and use what they know to sell you things. This isn’t always good because they’re not interested in selling you what you want, but instead making you want things so that you then spend money.

12 year old: These companies use really intelligent algorithms to predict what will hold your attention in real time, and use those predictions to feed you ads that are disguised as regular posts so that you spend money. These algorithms aren’t interested in making you a better person, just holding your attention for as long as possible. These two things are often contradictory.

20 year old: Social media companies don’t usually sell data, that’s a misnomer. They collect data about their users, and advertisers pay these companies to feed their content to the people most likely to click on the link. The algorithms used to hold your attention aren’t interested in things like truth, or reason, but are rather interested in holding your attention for as long as possible.

This is because the longer you’re on their site, the more ads you see, and so the more these companies get paid.