How the heck does one spend $539.5M per year running something like Slack, that's what I'd like to know. It just doesn't seem that complicated of a service.
Their operating expenses are divided in 3: Research and development, Sales and marketing and General and administrative.
They also describe a bit the evolution of these under the Risk Section, beneath "We have a history of net losses, we anticipate increasing operating expenses in the future, and we may not be able to achieve and, if achieved, maintain profitability."