The PC maker's U.S. outlet store has been on Twitter since June 2007, posting specials and responding to customer questions. And according to the Direct2Dell blog, sales generated by those regular tweets have generated more than $3 million in sales.

Broken down, that's $2 million in Dell Outlet sales, plus another $1 million from customers who entered the Dell site via Twitter, but who wound up buying hardware elsewhere on the site. Dell Outlet uses Twitters to promote clearance sales, coupons, and other discount-related fodder.

Dell's success lends credence to the argument that Twitter is much more than a wasteland of shallow, "What are you doing?" posts. It also may provide a much-needed business model for Twitter by giving the social networking site a valid reason to charge for corporate accounts.

Dell's success proves that Twitter can be an effective marketing tool. Even better for Dell, its followers often re-tweet the vendor's deals to their followers, thereby boosting the reach of Dell's pitches.

It remains to be seen, however, if other businesses can repeat Dell's success. The PC maker's @DellOutlet account has more than 600,000 followers, which I guess shows that Twitter users are willing to put up with marketing drivel if they perceive a clear benefit--in this case, some pretty good bargains. (Dell has other Twitter accounts too, including those for its small business, home, and international outlets.)