It is said that the country is run by a small, rich cadre of people, to which none of us belong. Scott Adams suggests that the big decisions are made by the congress, the small ones are made by the bureaucracy, and everything in between is the realm of Mark Zuckerberg and 3 or 4 of his peers.
Check his reasoning here.
I think that as long as Facebook/Google/Twitter can control the flow of ideas without making it overly obvious, they can probably get away with it. This approach worked for the MSM for years. Once it got to the point where they couldn't get away with it, the deal collapsed.
Give it a listen.