Microsoft said users would embrace Teams because it had strong encryption and global support and worked seamlessly with software they already used, like Excel. “We think customers value coherence,” said Bryan Goode, the general manager of Office 365 at Microsoft.

Born From Games

Slack is the product of Mr. Butterfield’s second failed attempt to make a video game. His first, called Game Neverending, had a photo-sharing feature that became more popular than the game. It became Flickr, which was sold to Yahoo for $25 million in 2005.

In 2011, he introduced another game, Glitch, in which people cooperated to build a shared world. (Misbehaving players got a timeout.) At its height, the game was costing the company $500,000 a month and bringing in $30,000, so Mr. Butterfield shuttered it in late 2012. A few employees stayed on to build out the messaging platform that Glitch engineers had used to talk among themselves. That project became Slack.

Slack was officially introduced in February 2014 and today has five million daily users, up from four million in October. Most users are on a free version, but about 1.5 million pay Slack $6.50 to $12.50 a month for features like message storage and search. The 800-person company is on track to generate more than $200 million in revenue this year, but it is not profitable.

Initially popular with small teams of software engineers who wanted to work remotely without cumbersome videoconferences or email, Slack was not designed for big companies. Users chat in “channels,” typically organized around a few well-defined tasks.

“The overhead of working among different offices becomes less of an issue” with Slack, said Nick Coronges, chief technology officer at R/GA, a 2,000-person design and marketing agency with offices in several cities.

Collaboration software like Slack is not new. Google’s Wave, which started in 2009 and shut down in 2012, was supposed to replace email with a messaging tool, but conversations were too hard to follow and track. Atlassian, an Australian software company, released a team collaboration platform in 2004 and acquired Slack’s nearest start-up rival, HipChat, in 2012.