how should you respond to the times when someone 'invalidates' you

there's times i've posted a comment on a video, or published a post on social media, me just stating what i believe to be true....then someone else comes back at me with a reply saying ' no that's not true blah de blah ' and trying to undermine my comment or discredit it? in other words ' invalidate' me.

and in those times, i have felt a bit annoyed, but also aware that that other person is entitled to their opinion? but not when they're trying to discredit my comment and make me out to be untruthful or make me look stupid....it is invalidation from somebody that.

how should you respond to that do you think? has it happened to you at times?