More and more similar solutions already have this feature (for ex. ECamm Live, Be.Live, StreamYard).
During the live stream on Facebook, viewers give comments on Facebook. The host can see these comments in the Studio and can select one to project it on the selected screen (just as a lower third).
It appears with a nice design and everybody can visually see the comment. Invited guests can reply better, it give a lot better visual experience to the show.
Would be a great benefit to add this feature.