diaspora* (pod)
One of the first decentralized social networks (2010), conceived as a privacy-friendly Facebook alternative. Servers are called "pods" and federate with each other. A predecessor to today's Fediverse, but it uses its own protocol, not ActivityPub.
History: started by four NYU students after a viral 2010 crowdfunding campaign (a reaction against Facebook's privacy problems). Innovation: "aspects" — group contacts and share selectively (predating Google+ "circles"). Decentralized: choose or host a pod, federate with the rest. It uses the diaspora protocol (not ActivityPub) → doesn't federate directly with Mastodon, which isolated it when the Fediverse standardized on ActivityPub. Community-run since the founders left. Today small but still active. Historically important as proof that decentralized social networks can be built — it paved the way for GNU Social, Mastodon, and others.