How to Build a Successful Internal Communication System

Slack was built on IRC plus a whole bunch of stuff that we had done. We tried Yammer in 2010 and it lasted about six weeks before we realized now we had another thing to check. Once teams start using Slack, they essentially don't stop. People were happy to pay money for it. So by the time that we had to go back to raise more money because we had spent a million and a half out of that $5 million, it was a quarter of a billion dollar valuation.