I've always viewed this argument as a very poor one. This essentially mandates that all very successful online enterprises become government arms. When does Instragram/Twitter/Facebook, once made a "public platform", become free of these new responsibilities? Is Twitter free to shut down tomorrow once they have public fiduciary responsibilities?
I'm all in favor of additional regulation to encourage/reward/enforce "openness", especially with respect to data originating from users (photos, ...). But saying these companies have any responsibility to the social good is pure foolishness.