Email and Mastodon are existing decentralized social networks. If you want to call something Web3, it's going to need to be more than just "decentralized".

technically they are federated, not decentralized. Gmail is a centralized, federated node. Think like a federation of states where each state is its own centralized, independent unit.

the newer definition of "decentralization" is more akin to anarchy. Collectivism vs individualism.

Email is not "technically federated". It's de facto federated. You can run your own email server on your home computer.

Anything less centralized than email isn't going to be "Web3" because it's not going to be a web at all. For the purposes of the internet, there is no functional difference between federated and decentralized. Two strangers must have a way of contacting each other -- something like an IP address at a minimum. That's a centralization point right there.

In the case of the web, DNS is going to be the main way that things are centralized.

Blockchain is similarly federated -- it just has a way to make sure that each node has a mutually agreed upon state. That doesn't make it decentralized, it just makes it redundant and distributed.

I am trying to imagine a social network like that. How would it be different?

A social network is the place to gain clout, so high profile people would continue giving away their IP for free.

The network itself could still display context-aware advertising.

Isn't that just the late 90s internet? Make your geocities page with your info, link to whoever else's pages you want, use email or chat to communicate with them. HTTP, SMTP, IRC...