Slack, Stewart Butterfield’s Collaboration Software Startup, Has Raised $42.75M

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Big news today for Stewart Butterfield and Slack, his enterprise collaboration app. It’s raised $42.75 million, led by new investor The Social+Capital Partnership, with the participation of existing Tiny Speck investors Accel Partners and Andreessen Horowitz.

We first reported news of the funding yesterday, citing sources that said Slack was raising “$30 million plus.” Now it is official. This represents the biggest investment from Social+Capital to date.

Slack has come a long way since launching 10 weeks ago out of limited beta. Competing against the likes of Yammer and Convo, it says it now has 60,000 daily users and 15,000 paid seats.

It has picked up a number of high-profile users in that time, including Rdio, Medium, Soundcloud, Heroku, Lonely Planet and BuzzFeed. The main idea with this round is to scale that usage higher.

As part of the new round, Mamoon Hamid from Social+Capital has been appointed to the company’s board of directors, where he joins Andrew Braccia (Accel), John O’Farrell (a16z), and Butterfield.

Yelp’s Jeremy Stoppelman, Squarespace’s Anthony Caselena, and Stripe’s John and Patrick Collison, are all new angels participating in this round, along with existing investors from earlier rounds, which include LinkedIn CEO Jeff Weiner, Google VP Bradley Horowitz, former Groupon COO Rob Solomon, and Twitter co-founder Biz Stone.