I'm not even sure what it means to 'exit Australia'. We're talking a website here.
Additionally, Australia would probably come down hard on any attempts for Facebook to market or solicit customers within their nation.
If a government of a country defines a website illegal, then that website simply will not be accessible from that country, end of story.
The US, despite leading the charge vis-a-vis the MPAA, has speech laws that make website blocking and censorship extremely difficult.
Facebook Ireland operates Facebook in Australia.
https://www.smh.com.au/business/companies/how-facebook-austr...