I think viewing Mastodon the software through this lens is the wrong approach. the real special sauce is the ActivityPub protocol.

The protocol was given a well informed comment in the Thoughtworks Radar a few months ago (assess means check it out):

With the current upheaval in the micro-blogging platform space, the ActivityPub protocol is gaining prominence. ActivityPub is an open protocol for sharing information such as posts, publications and dates. It can be used to implement a social media platform, but the key benefit is that it delivers interoperability between different social media platforms. We expect ActivityPub will play a significant role in this space, but we’re mentioning it here because we’re intrigued by the possibilities beyond the obvious use cases in social media. An example is ActivityPub support for merge requests, recently proposed for GitLab.