I've just released a new private message module for Drupal 8. The module is currently in beta version, and needs some work for a full release. If anyone is interested in assisting in the remaining features for a full release, more information is available on the module page. https://www.drupal.org/project/private_message

This module will be a full-feature, fully-extensible, fully customizable private messaging system. The core is there, it just needs some additional work to get it ready for a full release.