Is there a social share plugin for wpforo?
All social sharing plugins should work on forum pages. Please read this support post: https://wpforo.com/community/how-to-and-troubleshooting-2/share-buttons/#post-6717
Could you give an example of a site that has this feature? I could not implement social sharing for each topic on our forum site - https://negroschronicle.com
Another question: Is there a way to add social sharing button on each reply header?
All sharing plugins add share button on all pages. Before forum area, after forum area or on sides. There is no sharing plugin which adds sharing button in forum area. So if you're on a topic page you can share topic page, if you're on forum page you can share the forum page.