Facebook has resolved the issue that was preventing Share links from being added to Posts on their service. All users should again see the Share links on their posts.
At this time, this is not something that can be added.
When Facebook released their new Graph API, and actively encouraged developers to use it, they broke the ability for services to attach custom links, such as "Share," to posts. (Facebook bug can be viewed here.) Contrary to popular belief, "Share" is not automatically added to posts by Facebook, though many developers, including those at dlvr.it, wish it was.