I think your best bet here is Nostr (Notes and Other Stuff Transmitted by Relays):
https://nostr.com https://github.com/nostr-protocol/nostrNostr isn't a federated platform like Mastodon or Lemmy, it's more similar to the AT protocol created by Bluesky, whilst being far simpler to understand and write apps using it. The nostr protocol is defined by a series of NIPs (Nostr implementation possibilites https://github.com/nostr-protocol/nips), the most basic of which can be implemented in a client or a relay in 50-100 lines of code in any modern programming language.
Each user runs a client, anyone can write a relay or run any of hundreds of existing implementations, both clients and relays can choose to support a number of NIPs. Users have a public-private keypair, and distribute notes to relays signed with their private key, which are verified by relays. Clients subscribe via websockets to any number of relays (I usually have 20-30), and receive notes from all users on those relays' databases, or filtered by the public keys of the users you're following. Relays for the most part don't communicate with each other. If you're ever blocked or banned from a relay, you'll still be able to have your notes seen as long as you have at least one relay in common with anyone who wants to see them. I run my own as well for extra resiliency.
At the moment there's ~50 standardised NIPs, which add features like likes, zaps (bitcoin tips for notes), user status, post expiration, mentions, search, DMs, and public chats. Nearly all of these are supported by popular clients and relays. While nostr is primarily used for social media at the moment, it's already possible to build upon as a protocol for pretty much any online service.
The total active user count on most public relays I'd estimate is somewhere around 500k to a million, though the nature of the protocol makes it impossible to estimate its true size. The perceived community on most relays before following anyone frankly can get pretty cancerous, mainly due to a lot of clients sorting notes by new by default, so I can only hope to high heaven it'll improve as it grows.
Though like any new non-centralised platform, it's more difficult to get started on for most non-technical users as they have to pick one of hundreds of clients to install, and requires caution to never leak your private key and be very wary of which clients you trust it with.