Fair use covers research, but creating a training database for your commercial product is distinctly different from research. They’re not publishing scientific papers, along with their data, which others can verify;

Since when is there a legal requirement to publish the results of your research?

They use other peoples’ work to profit. They should pay for it.

Sorry but that's just not how the world works. A big part of it is just plain practicality - how could you possibly find out who to pay? If I wanted to pay you one cent for the right to learn from things you've written on the fediverse, how would I even contact you? Or even find out who you are since I assume TWeaK isn't your real name. And how would I get the money to you?

Like it or not, a lot of value created doesn't get paid for. That's just the way the world works... and among other things, Fair Use codifies that fact into law.

Facebook steals the data of individuals. They should pay for that, too. We don’t exchange our data for access to their website (or for access to some 3rd party Facebook pays to put a pixel on)

Facebook isn't "stealing" that data. Third party websites voluntarily and put tracking pixels on their site with full awareness that visitors are going to be tracked. That's why they do it - the website operator is given access to all of the data facebook picks up. If you have a complaint, it should primarily be with the website operator especially if they don't ask the user for permission first (a lot of sites ask these days, I always say no personally. And run a browser extension that blocks it on sites that don't ask).