Mastodon is focused on posting on your personal account’s page and doesn’t really support groups AFAIK. Lemmy does have groups (communities) but doesn’t support personal posts. While I get that the two mimic their proprietary counterparts and this defines their functionality, as a self-hoster I’d like to host just one service that would combine the functionality of both Mastodon and Lemmy and potentially other Fediverse platforms.
So what is the platform that implements the most of ActivityPub features?


Hubzilla.
It’s the most powerful and feature-rich Fediverse server application ever, hands down. Also because it has features implemented that go vastly beyond what ActivityPub is capable of.
It was made by Friendica’s own creator by first forking Friendica twice over in 2011, then completely re-writing the second fork in 2012 and then expanding the re-written second fork in 2015.
While it was the first application to ever implement ActivityPub (July, 2017; two months before Mastodon), it has always been based on its own protocol, Zot, that was invented by Hubzilla’s own creator in 2011.
It’s Friendica, much more modular, with nomadic identity, with a staggering permissions system, expanded into a full-blown “social content management system” with all bells and whistles.
It’s basically Facebook meets WordPress meets Nextcloud meets a whole lot of other things.
Features:
Friendica has only got four advantages over Hubzilla. One, it supports the Mastodon client API (although that’s half-useless because Mastodon apps don’t even cover 20% of Friendica’s features). Two, it has mobile apps of its own, although stable releases only exist for Android; on Hubzilla, there’s only the Web UI for everyone anywhere. Three, it has even more connection options like being able to integrate a Bluesky account and “remote-control” it. Four, it’s easier to use; this is also because Hubzilla is highly modular with tons of optional, off-by-default stuff whereas just about everything is hard-coded to permanently on on Friendica.
Otherwise, Friendica is no real match for Hubzilla, and Bonfire is even less of a match.