Slack's valuation tops $7.1 billion as venture investors keep piling into the messaging app By: CNBC.com News August 21, 2018 at 09:00 AM EDT With 3 million paying users, Slack is now being valued by private investors at more than $7.1 billion. Read More >> Related Stocks: Alphabet-A Atlassian Corp Cisco Systems Microsoft T.Rowe Price Group