Not all big brands are using social media tools to the potential they could be. For this reason, it really sticks out when they do. There are plenty of small businesses you can learn a lot about ways to use social media from, but it is the big brands that have the truly wide reach. These are the ones that are easier to find, just because they're brands that you know.
Take Pizza Hut for example. The company seems to appear in the social media news circles fairly often. One time it was for posting a job opening for a Twittering position. Another time it was for using Twitter as a tool to drive its efforts to help feed the hungry.
Look at Ford. They utilize all kinds of different social media channels. They go where the people are, and they interact with them in different ways. They keep it at a human level. They look for trends and try to determine what people want. They use Twitter, Facebook, Flickr, YouTube, Scribd, Delicious, etc.
Mattel is the latest big brand to make headlines with social media efforts. The company just announced a new e-commerce site with social shopping features. WebProNews reported on that here.
You probably interact in some capacity with big brands on a daily basis, whether that is something as simple as getting a Coke out of the vending machine or running around the neighborhood in your Nikes. Find products you use. Find products you like and are interested in. Then see how they are using social media (if they are). This way you are placing yourself directly in the role of the customer, and you can evaluate exactly what you are getting out of their social media experience.
Since you can look at this from the customer's perspective, you can use that to determine strategies that you feel work, and those that you feel don't. It's not a bad way to find concepts you can apply to your own business or at least some that you can test.
Good for all brands
I think social media is good for all brands, big and small. The big guys and the little guys can use it to interact with customers and put them at a human level and the little guys especially can use it to do small promotions locally, get the word out about a special, etc. It's better than an email list!