Fediverse

Something being free and open source doesn't mean it's good

True. But it's verifiable.

It's FOSS and you've looked at the code and found it to live up to its claims of being secure?

Popular FOSS projects get audited all the time. Heck, there is even automated software to detect anomalies in code changes.

Auditability is the only reason why you can only really trust open source but not closed source. With proprietary software you'll always have to trust the developers to not do something shady and are competent enough. With open source you can simply verify it.

Also being open source is what usually makes popular FOSS more stable and secure than most closed counterparts. A LOT of people donate their work and since it's completely public, most want their contributions to be in good shape. If only a few or no other people see your code, you are tempted to write bad code a lot more. This of course is not always the case but more often than not.

Also in most developed countries it's illegal to purposefully introduce manipulated code. And I don't think most people would risk punishment for that if literally anybody could find it.

I'm trying to show that the particulars of why you like or prefer something matter.

Sure. But most people don't care about the details, unfortunately. In the case of messaging they just want to communicate. And if someone asks me, which platform I'd recommend I will always start with the most secure and private.