What is the best way to do online advertising but is free?

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I have thought about the most economic way to save money on advertising, or spread out the words about my business online. I just couldn't afford to pay any expense right now. I just want it to be going, and cut all the expense down, especially advertising. So, do you think youtube, or twitter, facebook, etc..... those social network should be the good media for advertising? Any other sources, or channels I can work it out to advertise my business online. Can anyone give me some good advice?