How on earth do you know how God is going to use you? You are commanded to speak the truth in season and out. It doesn't matter if no one will listen you (believe, I know). Cast your bread upon the water and it will not come back empty. That is a promise of God.
I have to admit that you're right on this point. I don't know how God is going to use me, but at the same time I also don't know if there's any point in trying to point out the Truth if the person I'm talking too isn't ready to hear it yet. I'll have to think and pray on this one, but you bring up a very good point and I haven't been even considering it.