Matt Mullenweg said Tumblr would support ActivityPub, the decentralized social-networking protocol used by services such as Mastodon.
Mullenweg, Automattic’s CEO, made the statement in November 2022, after Automattic had acquired Tumblr from Verizon in 2019. He described the goal as interoperability: Tumblr users and content could potentially connect with communities on other services that implement the same open standard.
ActivityPub is not a social-media website by itself. It is a protocol defining how platforms exchange posts, follows, replies, and other activity across independently operated servers. That network of connected services is commonly called the fediverse. Mastodon is its best-known example, while PeerTube and Pixelfed are other ActivityPub-based projects.
A useful distinction is that Mullenweg’s announcement was a plan, not proof that Tumblr had already completed federation. Reporting in 2023 said the integration was still being evaluated or worked on. The answer remains ActivityPub because that is the protocol he specifically named; Nostr, OStatus, and AT Protocol are separate projects.