Mastodon is itself a long-standing open-source, federated social network built on ActivityPub — so unlike Threads or X, it isn't a proprietary platform you need to "escape." It's included here because people researching decentralized social alternatives often compare it against other open protocols rather than against closed platforms.
Bluesky is the natural comparison: also open source, but built on the AT Protocol instead of ActivityPub, with a more centralized default network (though portable identity) versus Mastodon's fully federated server model. Pick Mastodon if you want a mature, federated network with many independently run servers; pick Bluesky if you prefer its algorithmic feed options and simpler onboarding.
FAQ
Is there a free, open-source alternative to Mastodon?
Mastodon is already open source and free. Bluesky is another open-source option (MIT-licensed) built on a different protocol, the AT Protocol instead of ActivityPub.
What's the best open-source alternative to Mastodon?
It depends on what you value: Mastodon's ActivityPub federation gives you thousands of independently run servers, while Bluesky's AT Protocol offers portable identity with a more unified default experience.
What are the top open-source alternatives to Mastodon?
Browse all open-source alternatives to Mastodon above, or explore the full Social category for more options.