You may want to make sure your old content is presented in the same template as your new content so it has the same sharing features.

People still may find their way to that old content, and they may want to share it if encouraged. Go back over old content, and look for stuff that is still relevant. You can update stories with new posts adding a fresher take, linking to the original. Encourage readers to follow the link and read the original article, which they may then link to themselves.