Stewart Butterfield, co-founder of Slack, the communication app for office workers that has been growing like a weed, does not mince his words when he discusses the changes to working life caused by smartphones and the spread of cloud software.

Slack represents one response to this trend. It is a chronological flow of team conversation akin to a group chat, or a Facebook news feed. Workers post their contributions — messages, links to websites or files — into this searchable repository, organised into channels.

A rival vision is represented by Dropbox, a cloud storage system which also claims to be seeing wide adoption in offices but which puts files, not a stream of chatter, at the centre.

This view holds that workers will congregate around more static repositories of information, communicating inside or alongside the files.

Mr Butterfield does not hesitate before delivering a characteristically curt put-down of Dropbox: “They’re probably just wrong.”

If the boss of the hottest new app to enter office life has flaws, a lack of certainty is not one of them.

Slack’s headlong growth suggests he has hit on something, though at less than two years old it is still early. Mr Butterfield says the service has been growing exponentially, adding new users at a rate of about 5 per cent a week. Slack says 1.1m people use its service each day and that it has 300,000 users covered by subscriptions.

“It’s a much more profound change than people realise,” Mr Butterfield says of the way mobile and cloud technologies are affecting work. “The endpoints are people — not documents, or files, or content, or ‘stuff’ in the generic sense. The number of people for whom data stored in the form of a file is the most important thing is getting smaller and smaller all the time.”

Thoughtful and intense, as well as flip and dismissive, Mr Butterfield is acerbic at one moment and liable to dive into an extended technical explanation the next.

If devising the formula for a successful viral internet service is like catching lightning in a bottle, he has the distinction of now having done it twice: Flickr, the photo-sharing site he co-founded in 2004 and sold to Yahoo the next year, was one of the first signs of the social media wave that followed.

Slack takes its own cue from social networking. But getting the formula correct involves much more than an aptitude for finding the right social chemistry, its boss says.

It is also about making use of what he calls the “profusion of software” inside companies. With more corporate functions being automated through cloud services — from keeping track of customer orders to monitoring IT systems — Slack is designed to integrate their output into a single stream of communication: a notification could be posted to the stream, for instance, each time a new order comes in or a customer tweets about a product.

That makes Slack a digital spine for a business, with a single search box for tracking both human- and machine-generated data.

The aim is to create “a positive feedback loop, where the more attention you pay to it, the more it becomes something that saves time in switching between multiple obligations”.

The formula has been a hit with venture capital investors as much as with users. Slack has already raised $340m in its short life, with the most recent funding round putting a valuation on it of $2.8bn — notable even in the current buoyant funding environment in Silicon Valley.

Asked why Slack has raised so much, Mr Butterfield says: “Because we could.” With more than a quarter of its users paying for the service, Slack is “pretty much even on a cash-flow basis already,” he says.

But stashing away money in the good times could become a competitive advantage in a bust. “I wouldn’t wish a crash on anybody, but the best-case scenario for us is that money becomes harder to get,” he says.

To justify the sky-high expectations, Mr Butterfield will have to prove that Slack is more than just the latest faddish office-messaging system. The challenge now is the one that faces all new enterprise software companies with a bright idea: to move fast enough to keep ahead of the copycats, while finding a way to steer between giant competitors that might see Slack as a rival and try to crush it.

The race to keep Slack’s user numbers growing exponentially turns on overcoming barriers to its adoption. A key product enhancement planned for later this year, for instance, would greatly simplify things for corporate IT departments that want to control the many different Slack group chats that may have sprung up inside their businesses. Slack is also adapting its service to meet different international sensitivities.

Avoiding head-on confrontation with the giants of the software industry, meanwhile, means not challenging their own businesses directly. “In this moment now, we have effectively no competition and we’re young and new enough that no one hates us yet,” Mr Butterfield says.

His plan for keeping it that way is to go broad rather than deep: find millions more users for the existing service, rather than add many new functions that suck in more and more of its users’ attention.

“People are paying us between $80 and $100 a year [per user]. If we can get 100m people to do that, that looks like a pretty good business,” he says.

The biggest danger of all, though, is that some new piece of software will do to Slack what it is trying to do to more established software companies: offer a service that is even more basic and easy to use, and that starts to generate network effects as groups of workers find it slots almost unnoticeably into their office lives.

“The thing I worry about most is someone coming up with something that is much, much simpler but delivers most of the value — like they find the magic formula so that you can get 80 per cent of the value of Slack but with only 20 per cent of the complexity.”