Interesting new(?) ActivityPub platform: Holos, the server runs on your own phone

https://holos.social/

A Mobile-First Federated Social Network

Connect to the fediverse through a relay server designed specifically for mobile devices. Join a decentralized network where you control your data, your privacy, and your social experience.

Unlike traditional social networks where your data lives on remote servers, Holos runs a complete ActivityPub server directly on your smartphone. Your posts, followers, and cryptographic keys stay on your device, giving you control and the ability to migrate between relays.