I am using a standard line "It's fake. In general you should consider anything you see on Facebook, Twitter, Whatsapp or the Internet to be false by default."

Engaging in refuting online garbage constantly is a painful waste of time, and keeping silent or blocking it just allows it to fester.

I'm hoping that this makes a difference, and I've used a trivial and non-threatening example of fakery (fake supermoon pics) to plant the idea that nothing online can be trusted, within my social circles.