Before Facebook and Twitter, consumers used good old word-of-mouth to tell their friends about good (or bad) experinces and products. Now, brands not only foster relationships with consumers by inviting candid feedback, but also encourage consumer-to-consumer conversation beyond their control. Mentions of products can be “hashtagged” and tracked across social media networks. These new outlets helped transform a once informal marketing tactic into a measurable campaign form. Here’s how it works.