Ok, so Twitter does seem to limit it's bans to fairly extreme cases (and just 'un-verifies' people they dislike but who avoid blatanly breaching the rules). Facebook can be more heavy handed in banning controversial stuff, but perhaps more out of incompetence/inconsistency than political agendas. YouTube these days seems rather aggressive with demonetisation - although less quick to ban - so maybe they're just trying to save money?...

But there's *definitely* places where 'wrongthinkers/dissenters' are very quickly banned/silenced - many subreddits right here included. Some will even pre-emptively ban you just for posting on a 'bad' subreddit (even if you posted there to argue/disagree with the 'bad' opinions)

For an almost comical example of an echo chamber, spend some time on ResetEra (gaming forum) and observe how quickly the banhammer lands when anybody even very mildly and politely tries to question the groupthink. It's one thing to ban the hatemongers, but it can quickly degenerate into an effort to enforcing ideological purity and ensure that the ideology is never openly questioned.