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And have they been shut down? Blocking access to a service through the regular channels doesn't shut it down.

For instance before the internet a Saudi dissident based in the UK used to send faxes to phones in Saudi Arabia that were critical of the Govt. I don't think the Saudi govt blocked access to phone calls from the UK on account of that.

If Brazilian enterprises are not allowed by law to trade with Twitter that does not amount to shutting it down. All you've done is block the usage of a service by most law abiding citizens which many of them probably rely on for all or a part of their living.

Twitter is operating under US law and Brazilian law only applies to their Brazilian subsidiary. If their Brazilian subsidiary has been has closed down because their parent company is not complying with Brazilian law then Brazilians are free to deal directly with the parent company under US law and jurisdiction.