The latest entrant into technology's billion-dollar club got there by mistake.
Slack is the outgrowth of a communication system that Stewart Butterfield built for his last product, a video game called Glitch. But there was one problem. Few people played Glitch—the game was a failure.
However, Butterfield and his team loved the product they'd built to communicate with one another, share files and search chat history, so they shut down Glitch and focused on Slack. Since launching the product in February, the San Francisco-based company has signed on 30,000 teams, including 73,000 paid individual users. Customers include Expedia, Pandora and Salesforce.com.
Read MoreThe game of venture capital
Slack announced Friday that it raised $120 million at a valuation, including the fresh capital, of $1.12 billion. Kleiner Perkins Caufield & Byers and Google Ventures led the financing, with participation from existing investors Andreessen Horowitz, Accel Partners and The Social+Capital Partnership.
Prior to the current funding, Slack and its predecessor had raised $60 million, with most of that coming in an April financing round.
In February, Marc Andreessen tweeted this about Slack: "I have never seen viral enterprise app takeoff like this before--all word of mouth." And then six months later he tweeted a chart showing Slack's rocket-ship growth from fewer than 16,000 daily active users to over 128,000, calling it "unprecedented."