>whether its effective is another debate

Why?

For one thing, they have agreements between whoever provides them service, and if they have not been taken down, it means their business partners feel that they uphold their end of the bargain. Or they run their own infra so they only answer to themselves.

So did Parler. And whether it was effective was another debate as you said. But upon my causal visits to Parler I did not encounter a single call for violence. It seemed like it was as exceedingly rare as anywhere else.

Parler had moderation too. The difference is that Facebook, Instagram, and Twitter's moderation lean left, and Parler's leaned right.

I feel like you may have a right-skewed view of this issue. If you look at it globally it may be different, and I don't know where you're from. Here in the United States (where all four sites are headquartered) sites like Facebook/Twitter are moderated somewhat right leaning for their role in aiding right-leaning former president Donald Trump in 2016. He lost their support later due to his abuse of power but that's a bit out of scope of a HN comment.