> If Facebook was worse than the web for users, they'd flock to the web.

People go to these platforms for a reason, my point is that the reason isn't because they are walled off. It seems like you are arguing that the chief (if inadvertent) benefit to users of a walled garden is that users don't have to deal with undesirable behavior because access to the platform is restricted by a login wall. This isn't how I would understand a service being "walled off" - Hacker News is not a walled garden even though I need an account to access some of its features. The important distinction is that most (all?) of the content on Hacker News can be accessed without an account. Facebook, Pinterest, and Quora are examples of services going the other way - they lock down content, not for the benefit of the users, but for the benefit of themselves. They save on not having to serve the content to unregistered users, keep the content on their platform, and encourage unregistered users to sign up.

The chief benefit of the open web was always permissive read access, not permissive write access.

But a lot of times the "benefit" is simply that people they know are there. Which is the problem with all these open platforms like Mastodon. People can get the argument that it is better in theory to use an open platform. But nobody wants to use a social platform alone.

A good enough service and smart enough marketing can solve the bootstrap problem. Companies do it all the time!