Seems pretty straightforward, if Canada wants some news to be spread via Facebook, they could create a legislative niche that when a national emergency is declared, links to news will not be taxed.

Or they could have a clearing house of certain links that don't get taxed.

Facebook's responding to market incentives, as they're supposed to do. So you apply a tax and Facebook's opting out of the tax as is allowed by the law. You can't then force them to do the voluntary thing to pay the tax.

Governments often apply the same logic to things they don't want people to do. Like smoking. The sin attached to smoking is deliberately and clearly created to keep people from smoking. This is exactly the same as the government putting a 2000% tax on cigarettes and then complaining people aren't smoking anymore.