Are you sure? Maybe you did not change the mind of the one's your debating with, but there are people watching (who never reply) who's mind you might have changed a little.
In real live, when I have a conversation with one person in a group that person will mostly get in the defensive mode, but when I looked around, I could see some people become very focused and not saying anything.
Now I can't read minds, but I had the impression they where listening and where thinking.
You never know who's listening to thing's they never heard before.