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More than a decade ago, Stewart Butterfield set out to create an ambitious online game with his wife at the time, Caterina Fake. It didn't go anywhere. But the photo-sharing service they invented on the side, Flickr, turned out to be a keeper. History repeated when Butterfield again tried to create an online game and ended up producing Slack, a collaborative messaging platform for business use that has become a phenomenon since its launch in August 2013.

Slack entered a crowded category, competing against established players such as HipChat, Flowdock, and Campfire. It immediately stood out, both for its potent features (you can tell it to notify you whenever a particular keyword gets mentioned) and genial, quirky personality (when you log in, it greets you with deep thoughts such as, "The mystery of life isn’t a problem to solve, but a reality to experience").

Only 24 hours after Slack launched, 8,000 companies had signed up. Today, the service has 360,000 daily users, spread out among 40,000 teams; the average user is logged on for 10 hours a day. "At this point, maybe the majority of venture-backed Bay Area tech startups use Slack," says Butterfield. "But it’s obviously interesting for us to get beyond that."

Slack still feels like a powerful instant-messaging client, but much of its potential stems from the institutional knowledge that companies build up as they use it: "Every discussion, every decision, every link, every document," Butterfield says. "The experience of being able to search back over all your team’s communications is super valuable." That helps explain how Slack itself got so valuable so fast. The $120 million in venture funding it raised last fall puts it in the exclusive club of enterprise-oriented startups with valuations of $1 billion and above.