If you have something that's "just like youtube, but we don't delete your stuff" it shouldn't be surprising that the vast amount of content is stuff that was deleted from youtube

I think this effect becomes a strategic defensive moat for established social media websites. Anybody who tries to compete with established players will find that their earliest adopters are the most unsavory sort, banned by other sites looking for a new home. This is the more insidious side of 'network effects.'

This was exactly my point and I am disappointed to see that I was downvoted for saying so. I am still surprised at just how swiftly Gab, for example, spread into the Fediverse.

Some of the early adopters are definitely unsavory, yet that content is what new users are going to see. Peertube/etc are still too small to protect against it, I fear.

Founding cohorts matter.

Early online worlds were (often, not always), academics and techies. Even that was often hard going to generate larger appeal. Facebook was Literally Harvard.

Random malcontents have an even higher social hill to climb.

An elite founding cohort tends to regress to the mean. An unsavory cohort ... tends to further radicalise or collapse entirely.

Fandom/hobby groups are more sustainable and often avoid pathological evolution, but rarely seem to develop mass appeal.

And all this before adding financial or business pressures.