Is it my responsibility to refute misinformation in social situations? Seldom does a day pass without a juicy piece of misinformation catching my eye. Generally these are the regular thoroughly debunked myths. Be it that vaccines cause autism, or humans cannot possibly influence the climate system since 'we're so small compared to the Sun'. Sure, I utter to myself after a four pi-radian eyeroll.
Is that all I should be doing?

Voices in my head

A second-year chemistry student hardly possesses sufficient experience in climatology or immunology.

But established basic scientific facts do not need a PhD to communicate. After all, science is not about arguments from authority. Rather it bases upon peer-reviewed research published in reputable journals. As far as I restrict myself to citable argumentation, I should be fine.

Good luck with that! This individual has already made up their mind. Any mention of a myth will have them in a defensive position, ready to turn on caps lock. Then you will only reinforce the erroneous beliefs you set out to disprove.

Fine, I shall not proceed.


(repeat)

Ultimately, these are my friends. Former training buddies, in some cases relatives, or even an occasional lecturer. Sitting idly has not worked once. If anything, one myth seems to build on the other. A guy who thought Coca-Cola Zero contained no calories now shares blogs which propose that smoking is beneficial. Not a true story (yet) but you get the idea.
Is it my responsibility as a student of science to embark on this quest? Or is the exercise in vain, at best up to representatives of the corresponding fields of study?
By the way, there is no evidence that the Earth is round... (Yes, a bad joke I left for last.)
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.