Like many other business apps, this one is all about collaboration, meaning it lets teams of coworkers chat, send private messages, or share files and pictures.
But it's going viral right now because it also lets them search. They can easily find snippets of conversations and files, and, more importantly, they can use it to search through a bunch of other popular cloud apps that the team might use together like Google Drive, Dropbox, GitHub, Twitter, Asana, and others.
Investor Marc Andreessen tweeted an image that shows Slack going from about 4,000 daily users in January to somewhere in the 15,000-user neighborhood today. The app, which works on phones and browsers, officially launched in August.
Andreessen has good reason to praise Slack. He and his VC firm Andreessen Horowitz helped fund Butterfield's company, Tiny Speck, which has raised $17.2 million in three rounds so far, according to Crunchbase.
And Tiny Speck's first app failed. It was an online game called Glitch. Butterfield shut down Glitch in 2012 and laid off most of Tiny Speck's 30 employees, TechCrunch's Ingrid Lunden reported at the time.
Slack is a reboot for the company.
To be sure, 15,000 users isn't bad considering that the company doesn't even have a Facebook page yet, just a Twitter account. It really hasn't done much marketing beyond posting the love tweets it gets from users like this one:
But Slack also faces stiff competition. There are a ton of collaboration apps already out there, each with their own schtick: Yammer, Chatter, Work.com, Basecamp, Tibbr, Tomfoolery, and the list goes on. Even diamond billionaire Fred Mouawad launched a similar app last year called TaskWorld.
Plus, viral popularity doesn't always translate into commercial success. For instance, 37Signals, which makes a popular chat app called Campfire and a popular project management app, Basecamp, said last week it was dropping Campfire and its other apps to concentrate on Basecamp, its one real commercial success.