You’ve probably heard about Slack’s exponential growth. And you may have read how the internal-communication platform — now just two years old — is already used by more than 30,000 teams and valued at over $1 billion. But have you visited its Twitter Wall of Love? These tweets are real, and they're the stuff of founders’ dreams. And yet Slack hasn’t run any big integrated marketing campaigns — they don't have an elaborate email strategy or buy million-dollar billboards. (In fact, they hit those user numbers without a CMO.) So how did the company not only launch with enviable momentum, but so quickly win users' hearts? If there’s one theme that emerges when founder Stewart Butterfield talks about Slack's success, it’s that the company made customer feedback the epicenter of its efforts.