Correcting someone in a public venue isn't for the audience of one that is spouting stupid things, it's for the rest of the world that is reading the thread. So a fact-based correction that causes more stupid to be posted is doing it's job, really.
If the response happens to reference the facts that were posted, well, now you've got a conversation going.
I would also like to point out that stupid is in the eye of the beholder sometimes. Something can sound stupid on first blush yet turn out to be true.