LPT - When engaging in a debate, especially online, use the "reporter method" - quote or paraphrase a source to make your counterpoint and ask them to respond. Your "question" will make your point.

This method avoids others feeling like you are personally attacking them and getting emotional.

Example:

Them: "I believe that X is true"

You: No, everyone knows Y is true, how dumb are you?

You: "How do you respond to the recent NY Times article that says that actually Y is true?"