Last I saw, hyperlinks didn't "provide a client to server API for creating, updating and deleting content, as well as a federated server to server API for delivering notifications and content".

Are there any practical applications of this protocol on the wild, or on paper? This server-to-server communication reminds me of email in a good way. It could open a door for websites to talk to each other without Facebook or Twitter chaperoning the whole thing.

Federated microblogging: https://github.com/tootsuite/mastodon

Distributed video streaming: https://github.com/Chocobozzz/PeerTube

I don't think Mastodon has implemented ActivityPub yet, does it?

Mastodon has switched to ActivityPub already