Mastodon

The self-hosted, federated microblogging server behind the biggest corner of the fediverse.

Best for: Communities or brands who want a federated, algorithm-free social network they fully own, moderate, and can interoperate with the wider fediverse.
Skip if: You want one centralized platform under your control - federation means no single owner over discovery, moderation, or the network at large.

Mastodon is an open-source (AGPLv3) social networking server built on Rails and React, implementing the ActivityPub protocol so any self-hosted instance federates with the wider fediverse. Each instance sets its own moderation rules while users can follow and interact with accounts on any other Mastodon (or ActivityPub-compatible) server. It's free software sustained by a non-profit and donations, not a hosted SaaS product.

Pros

Interoperates with the entire ActivityPub fediverse instead of being an isolated island
Fully open source under AGPLv3, no vendor, funded as a non-profit rather than by ads or data sales
Mature per-instance moderation tooling and content policy controls

Cons

AGPLv3 requires publishing source for any modified version you run as a network service
Self-hosting means Rails + Postgres + Redis + Sidekiq ops plus federation-specific spam/abuse moderation
No built-in monetization path - sustaining an instance is entirely on the operator