From their headquarters in San Francisco, Slack's 63 employees are developing technology that they think could make company-wide e-mails obsolete.

Slack pulls disparate applications - customer service programs, e-mail apps and social media Web sites, among others - into a single platform, which employees can also use to message each other directly. Instead of toggling between Google's Gmail service and Twitter, a user might receive notifications through Slack, for instance.

Chief executive and co-founder Stewart Butterfield, who previously co-founded photo-sharing site Flickr, is chasing the business technology market in search of companies willing to take a chance on his young start-up's product.

A growing number are willing, and so are investors. In the first eight months since Slack's public launch, it has racked up about 73,000 daily active users from thousands of companies. Current customers who use Slack for internal communication include Expedia, Buzzfeed, Pandora, PayPal, Urban Outfitters, Yelp, eBay, Tumblr, HBO, Vox Media, Gawker Media, Slate, Salesforce, Nordstrom, and Airbnb.

Last week, Slack announced it had raised $120 million from investors, including Kleiner Perkins Caufield & Byers and Google Ventures, in addition to a previous $60 million funding round with participation from Andreessen Horowitz, Accel Partners and the Social+Capital Partnership.

As Slack continues to raised funds, its software is evolving - but it isn't yet perfect, said Stuart Symington, product manager at Fluencia, a Rosslyn-based software company that has used Slack since July. Fluencia, which develops an online Spanish learning service, pays $6.67 per month per user for each of its 20 employees.

"It's totally replaced e-mail for us," Symington said. At Fluencia, employees use it to message each other individually or to communicate with project teams, he said - a few months ago, they would have used Gmail and its instant messaging service Gchat, he said.

But he noted that Slack's software doesn't always integrate seamlessly with outside applications, even if Slack's team is very responsive to feedback through Twitter and e-mail. For instance, Fluencia's team tried to add Internet cloud-based project management system Pivotal Tracker to Slack, but the integration was buggy (often notifications failed to show up in Slack), so employees access Pivotal Tracker separately, he said.

There have also been outages. Last month, Slack noted on its blog two separate incidents in which users were unable to connect to the service for periods that last from minutes to a couple of hours.

Despite the bugs, Fluencia's small team is willing to try out new products, especially if they are mostly for internal use, Symington said.

"It's not a huge risk to try something out for a week," he said.

Companies such as Fluencia represent a large chunk of Slack's customers - young, tech-savvy start-ups in creative industries that used to rely on free, "ad hoc" applications to message each other - Gmail, Facebook Chat, or Skype Chat, Butterfield said.

"When you start a company, you don't necessarily think you need to choose a system for internal communication," he said, though he noted that companies often decide on customer relationship management software, or inventory management systems, before they begin operations.

The bigger challenge for Slack is reaching large companies, Butterfield said - though he noted that the company is working on scaling the software up for bigger customers so that it can manage several different teams of Slack users, perhaps divided by department. Sometime next year, Slack plans to introduce an "enterprise plan" that will cost large companies between $49 and $99 per user per month.

Though large business customers look carefully at the security implications of a new product like Slack's before they commit to using it for large teams, "small disruptive vendors" like Slack can become very viable quickly, Forrester Research analyst Rob Koplowitz said in an e-mail, especially when the service is "easily provisioned by small groups and grows virally."

Other tech start-ups, such as Internet cloud storage companies Box and Dropbox, which are now used widely by business customers, "were questioned when they first started popping up in the enterprise, but they've responded quite well."