In short, anything you publish here will, by design, be broadcast all around to whatever service is interested in listening. Collecting data about fediverse users is as easy as setting up a federated service designed to collect data, and observe what comes in. In some countries there are regulations for what kind of data you’re allowed to store, but you could always just go somewhere else and do it.

Nobody is going to buy user data from sh.itjust.works or any other service because why the hell would they do that when they can just collect everything for free.

This is not unlike everything else on the Internet. If you publish something, it can be used. Maybe not legally in all countries, but as we have learned from the AI revolution nobody really cares about legality anyway.

Nothing you post here, or anywhere else on the internet, is private. It’s all public, and if companies find a way of profiting off it chances are that they will. If they can’t do it legally in the US they will do so somewhere else. The only way of avoiding it is by not publishing in open forums.

If you’re worried about your user data, the best you can do is probably to jump around between different accounts.