Slack Technologies Inc. /zigman2/quotes/212180539/composite WORK -2.41% shares rocketed 13% in Thursday morning trading after Chief Executive Stuart Butterfield tweeted new updates on the company's user-growth momentum. Slack had 12.5 million "simultaneously connected" users Wednesday, up from 12 million on Monday. The metric is different than the more traditional count of daily active users, which Slack and peers disclose periodically. As of September 2019, Slack had more than 12 million daily active users, and the company's user base has grown since then. On March 19, the company said it had added 7,000 paying customers since start of February, more than in all of prior quarter. Shares of workplace-communications companies have been resilient during the COVID-19 outbreak on the expectation that more people are working from home and need digital tools to stay connected to their coworkers. Zoom Video Communications Inc. shares /zigman2/quotes/211319643/composite ZM +1.01% have added 35% over the past month, as Slack's stock has added 11% and the S&P 500 /zigman2/quotes/210599714/realtime SPX +0.45% has dropped 19%.