Is it my responsibility to refute misinformation in social situations?
I think you should say more, but definitely don't respond all the time, and definitely don't be too harsh all the time. 

It's not black and white, and you have to judge every time if it's worth it, if it's your place to say something and if anybody benefits from you saying it.

Compare it too remarks about looks. You'd tell a friend he should wear another color, you'd tell a colleague that his sweater is inside out, you'd tell a stranger on the elevator that a label is sticking out, but internet remarks about how Britney looks nowadays are completely useless.