You said in your original comment:

> using Discourse for a private group and Twitter/Facebook for public groups

Even if what freeone3000 said about Mastodon is also all true about Discourse (I know nothing about Discourse, but I'll take your word for it), it's not true about Twitter/Facebook. And since Mastodon is aimed more at the public group use case, Mastodon is a useful replacement of Twitter/Facebook even by your own logic.

Not quite: Twitter and Facebook have _armies_ of community moderation employees at their disposal; the fediverse has volunteers.

No, the fediverse has at least one per instance, often more. I would even go as far saying the fedi has more mods per users. They're still only mods, for the instance they're on, though.

I have never seen pornography on Facebook.

I saw pornography of very dubious legality within seconds of opening a public Mastodon server during my first time checking out the fediverse. It was enough to convince me never to use Mastodon again.

And I saw pornography on Twitter, which obviously IS THE ting Mastodon copies, too. It totally depends on the instance's mods what they allow and yes, mastodon.social allows porn. Mastodon is not a social platform for catholics. If you see porn without NSFW Content Warning, you can report it.