These features have helped turn Slack into one of the fastest-growing business applications in history. After only a year in operation, Slack now serves about half a million workers every day as a partial replacement for email, instant messaging and face-to-face meetings. Its base of users is doubling every three months, according to Stewart Butterfield, Slack’s co-founder and chief executive. Mr. Butterfield predicts that by the end of the year, two million to three million workers around the world will be using Slack. (Slack is a private company, and its user numbers cannot be independently verified.)

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