Almost all the time, if someone's repeating crazy stuff I don't bother arguing, regardless of the social situation. If it's a friend who's sorta apathetic and uninformed and is freaked out by something they got emailed, sure, explain to them that it's overblown and they don't need to worry. But almost all the time the people repeating this stuff get a steady stream of it and it's a big part of their identity. You can't expect a productive reaction in those cases.
Also, I kind of lied when I said Facebook was my first exposure to this. Not necessarily rightwing emails, but you work with seniors, and it's like everything they know about the world they got through a game of telephone. Heard one old man explaining to another that the point of the 999 plan was to make sure rich people had to pay their share by including a sales tax.