One of the most packed presentations at the Web 2.0 Expo in San Francisco Wednesday was about how businesses should – and shouldn’t – go about using micro-blogging service Twitter.

But Sarah Milstein, a Web 2.0 strategist who has written a book on Twitter and who co-founded 20slides.com, a site that runs work-related workshops, warned in her presentation that companies shouldn’t just think of Twitter as another way to tout their own wares. “The biggest mistake a company can make is thinking this is a great place to push out information about itself,” said Milstein. “It turns out it isn’t a great place to do that at all.” Twitter, she warned, isn’t advertising — it’s an opt-in medium.

But the most important first step for a company, she said, is to use Twitter to listen. “Before you get on Twitter, keep an eye on what people are saying that might be unhappy. They are going to expect that you will be able to respond,” said Milstein.

Twitter, she said, can be a “canary in the coal mine” for companies to hear what their customers are saying about them before the problem explodes.