Never heard of the Mastodon project, sounds interesting.
Another project that OFN could potentially benefit from is the Diaspora Project which is a decentralized social network. They use a federation protocol to connect their pods (they call these their individual instances).
It is written in Ruby (available as Ruby Gem), open-source and focused around decentralization and privacy.
Here is the federation protocol specification of the Diaspora project:
https://diaspora.github.io/diaspora_federation/