Oh how wonderful it would be if Facebook were to pull out of Australia.

The bar would have to be incredibly low for the US to retaliate against Australia over this specifically, seeing as Australia is an incredibly strategic pattern to the US in the Asia pacific region, especially with China being a far greater existential threat to the US economy verses Facebook leaving the Australian market.

Edit: Keep in mind we're talking hypotheticals, Facebook only said it would stop sharing news articles.

The most extreme thing that will end up happening will be a US diplomat reaching out to Australia on behalf of Facebook.

As nations, we contain multitudes. The US fights Canada r.e. lumber, but at the same time cooperates across a broad range of other aspects of international relations. Australia being a strategic partner in many respects does not preclude trade disputes when it comes to their laws as they impact Facebook.

I'm not suggesting allied nations cannot have disagreements & voice them with each other, I'm saying it's unlikely for the US to "retaliate" against Australia like this, when something like this can be better resolved with peaceful cooperative diplomacy.

Again, in this case we're talking hypotheticals with Facebook pulling out of Australia, as they've only said they would stop showing news. It's not like Facebook would suddenly be unable to sustain operations in Australia because users cannot share news articles. If anything they're just playing politics to get a better deal for themselves.

If Facebook left Australia, I would because they choose to, not because they were forced to, just like no one is forcing Google to either.

Would the US really care about Facebook's concerns? Why?

Unlikely because newscorp controls the government in both Australia and the US

It doesn’t control either government, but they have the kind of symbiotic relationship that makes favours indistinguishable from control.