Slack is probably one of the most interesting branding, marketing and growth cases in the startups scene at the last decade.
Let's leave the numbers for a moment. to my opinion slack is a outstanding branding case study that every marketing student should learn from. They became a product that brand your startups. If you are NOT using slack, you're out of date, you're old, you don't know nothing about startups and you're not going to make it. Do you want to make it? to be an updated startup that knows what is important in life? use slack. and this is an amazing positioning. How did they do it? find out more here (mashable)