I've added this manually so it's working fine for me but I just wanted to point out that if you use the plugin to add the sharing buttons on posts they also show up under the forums. Not sure if there's a way to address that since I've seen it on other social plugins as well.
This can be done. We’ll be updating rtSocial soon and we’ll add such granular post type support in the future. It shows up just because forum topics and replies are just custom post types.