"Oh, you're friends with someone who says X? Welp, you must be a Bad User and your posts will receive a lot less visibility and the validation that you desire. But if you were to stop interacting with your Bad Friend, you could become a Good User again."
And oops, "Bad Friend" no longer gets invited to real-life parties because most people still consider FB the most convenient way to organize them and people only rarely remember to forward the invites.
You don’t have to tell 1 billion people in China not to say F%%% China”. You just punish a few and the rest fall in line.
Once you punish a few, the rest self censor.