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?
I like MBin. But I haven’t found toot functionality on PieFed.
It’s still early days for PieFed and microblogging. We got the ability to properly follow people a few weeks ago (in 1.7.0, I think). But a lot of stuff isn’t implemented yet. I’m not aware of a way to send out toots. And last time I tried replying to one, it didn’t show up on Mastodon. It’s slowly coming along though. If you’re planning to set up some Fedi service for the future, I think it’s worth keeping it on the radar. But as of now it’s still lacking in this regard, compared to for example MBin.