Mastodon, a free, open-source federated social network, based on the GNUsocial spec.

Mastodon's decentralized approach creates a social networking experience that pairs well with email, using the format <a rel="nofollow noreferrer" href="mailto:username@server.domain">username@server.domain</a> for accounts.

One of the primary concerns about adoption in Mastodon is that the operator of a server has access to user's private data; this seems like a good opportunity for Protonmail to extend the abilities provided under the blanket of customer trust already established, to create a secure endpoint for the social media network.

Also, I'd like nothing better than for my Mastodon handle to be, simply, my protonmail email address ;-)