Drupal 8.6 was released a couple weeks ago and it’s probably the most exciting release since Drupal 8.0. As you might know, new features are added with each minor release of Drupal 8 (e.g. between 8.5 and 8.6).
Backdrop CMS is based on the works of Drupal 7. It can be viewed as an extended version of Drupal 7 and branched off from Drupal 8 during the development cycle at a time when Drupal was getting completely refactored to include some Symfony2 framework and moving towards OOP while maintaining the Drupal UI and concepts.
Open Social is a distribution for building social communities & intranets. The distribution is positioned as the successor of Drupal Commons, taking full advantage of the new possibilities of Drupal 8.