NeoChat is a free open source @matrix messaging app for Linux desktop and mobile. You can follow at:

➡️ @neochat

It's made by @kde and the official NeoChat website is at apps.kde.org/neochat

Matrix is a free open source federated communications platform with support for messaging and calling. You can follow their official account at:

➡️ @matrix

The platform has many different apps available including the popular Element (formerly known as Riot). The official Matrix site is at matrix.org

Mastodon

A newer server operated by the Mastodon gGmbH non-profit