some notes from their S-1:
https://www.sec.gov/Archives/edgar/data/1764925/000162828019004786/slacks-1.htm
Financial statements show that Slack’s biggest expense by far is sales and marketing, which accounted for $233 million in fiscal 2019, equivalent to 58% of revenue. Slack has been spending heavily on advertising and “brand marketing.” Raising the profile of Slack’s brand name will be important when the company goes public, to ensure investors know what the company does.
• General and administrative costs doubled in the year to $112.7 million, which Slack attributed to expanded finance, accounting, legal and HR employees. That increase may reflect Slack’s preparation to become a public company.
• Slack’s net loss was roughly flat in the most recent year, at $138 million.
• The company’s paid customers use Slack on an average workday for more than 90 minutes.
• About 40% of its fiscal 2019 revenue came from just 575 paying customers, each of whom generated more than $100,000 in annual recurring revenue for Slack.