Far as I know, the streams repository has the only existing ActivityPub/ActivityStreams compliant event system. Hubzilla and Friendica also support events, but were originally built for earlier federated standards and need a few somewhat minor changes to support the current standard. All of these inter-operate today. Mobilizon doesn't federate events in a manner that works with any fediverse compliant implementations -- at all; and the developers have expressed no interest in cross-compatibility with other fediverse event implementations. So it's an interesting project, but caveat emptor.
A
