Mastodon is definitely not web3. It offers a world without Facebook/Twitter.

How would web3 improve that?

You don’t need a blockchain to do federation (I.e. Mastodon).

In a word, no. It's not concrete at all. How exactly would web3 technology solve all the issues these platforms are solving (including hosting, moderation, monetisation, dispute settling, software development etc.) we'll enough not only to be competitive, but to replace them ?

It’s not true enough

What problem would it solve that would make Twitter and FB obsolete and go away?

So could regulation

/s