In my ongoing quest to demystify social media, I thought this was a pretty succinct way to explain it. Good, successful social media really just boils down to providing good customer service.
Think about your business for a minute and how you treat the customers who call you, email you or talk to you in store. If they have questions, you answer them. If they have complaints, you address them and do whatever's necessary to rectify the situation.
The place a lot of businesses and brands get tripped up is in figuring out "what if people don't have questions or complaints?" Again, go back to what you'd do in real life: Be friendly, and be helpful.
Think about what you'd want to see if you were a customer. You use a product to make your life easier or more entertaining. Ads don't help you in either of those departments. Helpful information does: "Did you know X about Y?" "This link has a lot of good information on Z."
Before too long, you start to think of that brand as a resource. They're always sharing stuff you can use. That's the big "secret" to social media, and it's one of the oldest methods of success.
The next time you log into one of your branded social media accounts, ask a simple question: How can I be helpful to my customers? If you can answer it, and keep answering it, social success will follow.