In before bubble talk. This is absolutely insane, but I think it shows that investors have faith in the founding team to execute on their vision. This is a healthy reminder that companies are all evaluated on how people feel about them not, not the actual numbers: even publicly traded companies on the stock market. While I personally wouldn't invest in Slack at a 2.8B valuation, that doesn't mean other people don't think that they are worth that. Ultimately, Slack has the ability to execute and become a multi-billion dollar B2B company: I feel that they aren't that now. Other people disagree.