Unless you've changed it in your category settings, K2 settings or menu item settings, RSS feeds are universally enabled for K2 content. If a K2 category URL is "/blog", then to get the RSS feed for that blog simply append "?format=feed" so it becomes "/blog?format=feed".

Then grab these feeds and use a service like dlvr.it or IFTTT to post your content to social media.

K2 feeds are much more flexible than Joomla content's feeds because you're allowed to create any category combination as a menu item and then get a feed for that, unlike Joomla which in 2018 you still cannot create a menu item to multiple selected categories (so you can generate a feed from the menu item).