One of the problems with your argument, legacyln, is, that few things can really be done 'top-down'. Even tyrants rely on a subset of dedicated collaborators down there who are absolutely willing to spy and terrorize their neighbours either for a small material gain or out of ideological zealotry.
The experience of the world behind the Iron Curtain says that you must really be wary of whom you trust. The greatest force helping tyrants stay and grow is an unassuming balding man next door who collects all your shortcomings and informs on you regularly. Without a million people like him, every oppressive apparatus would be overwhelmed by the sheer amount of work needed to control an entire nation.
Whatever happens on Twitter each day really resembles the struggle sessions and witch hunts of yesteryear. At the end of the day, authoritarianism is carried on by will of a certain subset of the people, not just by authorities.