Now, thanks to momentum numbers released today -- Slack's first anniversary -- we know just how hot.
To wit: Slack reported that it has 500,000 daily users, with 135,000 paid accounts generating $12 million in annual revenue. At is current rate of growth, that breaks down to adding $1 million in annual revenue every 11 days. In the first six weeks of 2015 alone, Slack grew its business 135%.
That's insane growth, and Slack is now practically considered Silicon Valley royalty for being that rare startup that goes from nothing to major player in practically no time at all. Twenty people even proposed marriage to Slack on its Twitter feed, the company says.
"It's a little bit overwhelming," said Slack founder and CEO Stewart Butterfield. Butterfield had previously co-founded photo sharing site Flickr, which was sold to Yahoo in 2005.
The "overwhelming" part doesn't just apply to being thrust into the spotlight, Butterfield said: The company is growing so fast that it's devoting all its resources to keep up. This time in 2014, he said, Slack had 16 employees. Now it's got 105. The challenge is to keep that "lean startup" mentality that allows the company to quickly update the app and make it more appealing to a broader selection of enterprise users, while still growing enough to meet demand.
"One of our goals is to be as small as we can be without leaving opportunity on the floor," Butterfield said.
Obviously, the spotlight always fades. So Butterfield said Slack is rightly working hard to focus more on the business than the acclaim. Competitors like Hall and Atlassian HipChat are still out there doing business, and even if they lack Slack's fanatical fanbase, they can't be written off.
"Being on top just makes you more of a target," Butterfield said.
For the complete breakdown of Slack's first year, check out the company's official infographic below: