>Or, more cynical, reversed: I am convinced Mastodon will be a horrible platform if it grows to a size of e.g. twitter, reddit, and far, far worse if it grows to sizes of insta, tiktok, facebook etc.

The thing is Mastodon isn't a platform like twitter, reddit, etc, it's a decentralized network of platforms. A few instances might become popular (like mastodon.social) and become a default for newcomers but there will always be smaller instances doing their own thing.

But these instances are part of the platform. Or, a better term, part of the system.

So "doing their own thing" is very limited. Do "too much of it" and you'll be fediblocked. Too little of your own thing and you're essentially getting what the large instances dictate. Sure, your instance can e.g. block meta instances because it dislikes Facebook. But that doesn't isolate you from meta content completely - or even at all - it depends. If your instance wants that, it'll have to also block all instances that don't block meta. Basically defederating almost entirely.

And this doesn't even consider the numerous malicious practices that one can do, but aren't interesting currently due to the limited reach of this fediverse. Like scooping up expired domains of previous instances and using that to spam. Or like impersonating famous people and scamming users that way. Or like taking over accounts and spamming or scamming that way. Or even taking over entire instances by hacking the server or social engineering access. Etc. etc.

The fediverse has nothing technical that protects it against the real and expensive threats that "common social media platforms" face on a daily basis. Federation doesn't suddenly make any of these attacks more difficult. If anything, it makes it easier, because the instances themselves are ran by volunteers who in many cases will not have security budgets to protect against such attacks. The only thing that the fediverse has going for it, in this regard, is that the blast radius is smaller. The attack easier, the result contained. But it's not unthinkable that malware, fake news, or other malicious content can easily be spread this way.

You can do your own thing as much as you want, and choose with whom you federate, and others can do the same. That's just freedom of speech and freedom of association. Large instances aren't "dictating" anything to other instances. The point is you can't do that with Twitter or Facebook or even Reddit to the same degree. The fediverse will never have an Elon Musk trying to purge it of the "woke mind virus."

>And this doesn't even consider the numerous malicious practices that one can do (...)

Your argument here seems to be that because most instances are small, that makes them more prone to security issues. And that's true, but nothing you mention here is unique to the fediverse, and all of those risks are still greater with mainstream platforms even with their bigger security budgets and resources. People hack Facebook and Twitter all the time, and millions of people get their data stolen, and misinformation spreads like wildfire. In this regard being decentralized, smaller and able to defederate from malicious instances are features rather than flaws.

It is definitely a flaw in the fediverse that identity is tied to a url and so there's no way to validate an identity across instances, and it is trivial to impersonate someone by simply using their username elsewhere. I think Nostr got it more correct in this regard, and I don't know what could be done to mitigate this at a technical level.

But even then I think that at least the fediverse is a necessary step in the right direction.