It’s a massive money drain out of Australia. Look at how aggressively China protects its businesses by blocking Facebook etc from operating in its borders.
Because that's an overdetermined explanation. Their primary motivation is control of information, and that by itself is the only reason they need to ban Facebook. It's also why they ban a number of other sites that don't compete with Chinese companies.
Protecting Chinese social media companies from competition is at best a secondary motivation and it probably isn't even necessary -- Chinese internet users prefer Chinese social media for a variety of reasons, and I don't think Facebook is a significant competitive threat. Neither is Twitter.
On the other hand, China allowed Google to operate even though it might have been a strong competitor to Baidu. Google was only blocked when it refused to comply with censorship policies -- again demonstrating that control over information is the government's main concern.