Drupal simplenews doesn't subscribe the users registed via social networks
The reason users aren't signed up is due to how simplenews works. It does the work of newsletter signup by modifying the Drupal core registration form to capture the newsletters (or setting them automatically in case of the silent option) then takes that data to sign users up following account creation. 

For both HybridAuth &amp; FBOauth, they subvert this process with their own forms to do authentication with a 3rd party. Both HybridAuth &amp; FBOauth however, provide their own API hooks to take action upon user registration (hook_hybridauth_user_insert() &amp; hook_fboauth_user_save() respectively). You can implement their hooks in a custom module to do the same work HybridAuth does to signup users automatically.