You said in your original comment:
> using Discourse for a private group and Twitter/Facebook for public groups
Even if what freeone3000 said about Mastodon is also all true about Discourse (I know nothing about Discourse, but I'll take your word for it), it's not true about Twitter/Facebook. And since Mastodon is aimed more at the public group use case, Mastodon is a useful replacement of Twitter/Facebook even by your own logic.