Portfolio.com has a great article on how Dunkin' Donuts, and other businesses, uses Twitter to keep a pulse on their customer base as well as market (click here). Twitter is probably the most misused and misunderstood of new social media fads. Few businesses use Twitter, and even fewer use it correctly. I tend to agree with those who feel Twitter is just too "spammy" and full of false connections, but there are some really smart uses of Twitter, and Dunkin's Donuts found one of them.
Dunkin' Donuts is a business that, when it is working well, becomes a part of a consumer's day-to-day life. As such, it finds itself appearing in the vast stream of consciousness of Twitter. By being active on Twitter, Dunkin' Donuts can keep a pulse on how they are being perceived by their target base. They are able to converse with their customers, and have another FREE venue for announcing promotions and new products.
Almost any end-consumer business can benefit by this example. While Twitter is still not a good way, in my opinion, to grow your base or reach a lot of new customers, it is a great way to connect with and cultivate your current base.