If you're not familiar with different types of social etiquette, then you simply don't know enough to participate in discussions.

In this case, educate. Let's assume that you're correct... you have specific knowledge that could change the way the OP thinks about plastic. What exactly is your goal of participating in this conversation if it isn't to have a back and forth and learn from each other?

Maybe, by actually having a conversation with this person, you could better understand what misconceptions exist and how to combat them?