How would one explain to a Facebook addict why they should get the vaccine and that it's not a Bill Gates/NWO conspiracy to sterilise them?
Ask them to send you a photo of their smallpox vaccination scar, if they have one.
It’s not about convincing them that you’re right, because you can’t do that - they’re addicted to Facebook, so reason is clearly not operating as it is. It’s about changing how they weight their decision using factors you can control, like your physical presence and your Facebook wall.
(If you don’t know them well enough to see them in person, don’t bother, just cut them off and block them from seeing and commenting on anything you post.)