Who will enforce that responsibility? Who decides what's large enough?

> Letting one small group of people

What one small group of people? There are literally thousands of social media sites, and people are completely free to join or quit any one of them. If there were no competition in this space, I might be concerned. But just a few short years ago Facebook was unheard of and MySpace was the "monopoly".

Let customers choose. The customers are not one small group of people.