Workplace messaging platform Slack has become one of the hottest start-ups around on the basis of its astonishingly rapid user adoption and unicorn valuation. But now it's facing an onslaught of serious competition. 

Since launching in 2013, Slack has grown to an average of 4 million daily active users and 6 million weekly active users and was recently valued at $3.8 billion in a spring funding round. It also reached $100 million in annualized recurring revenue this year. 

Slack's impressive following and scale have generated hype around when it will announce an initial public offering, with some speculators suggesting it could be as early as 2017. However, Slack CEO Stewart Butterfield said an IPO isn't likely to occur for at least another few years, largely because the company's rapid growth has made it "harder to predict" where the company will be one year from now. 

But as investors await hints about a Slack IPO in the near future, the market for office communication platforms has become more and more crowded. 

Microsoft (MSFT) - Get Report earlier this month rolled out Teams, its own Slack-like competitor, while Facebook (FB) - Get Report in October launched Workplace, a business-focused social network and messaging service. Other collaboration platforms, such as Atlassian's (TEAM) - Get Report HipChat and Salesforce's (CRM) - Get Report Chatter, have been around for several years. 

Microsoft Teams is regarded as Slack's foremost competitor in the workplace communication sphere. Slack itself signaled concerns of competition when it took out a full-page advertisement addressing Microsoft Teams in the  New York Times

"We're genuinely excited to have some competition," the company wrote in the letter, which was published earlier this month on its website. "However, all this is harder than it looks. So, as you set out to build 'something just like it,' we want to give you some friendly advice."

The letter goes on to describe how Slack believes enterprise communication services must be centered on the end-user experience and third-party app integration, among other things. Ilya Fushman, a partner with Index Ventures who invests in Slack, said these qualities can be best summed up in one word: virality.

"The genius of Slack was it took the most viral types of content and gave people a platform where they can consume that," Fushman said by phone. 

Slack allows users to communicate via text and voice calls, but also has the option of emojis and GIFs. Features like these are what makes Slack a "cool company" in the enterprise software market that otherwise tends to be quite dull, said Jan Dawson, chief analyst at Jackdaw Research, in an email. 

That said, Microsoft should be viewed as a real threat to Slack's dominance in this market, said Morningstar Analyst Rodney Nelson. Slack may be the tech community's darling, but Microsoft has the potential for a giant captive user base from its 1 billion Microsoft Office customers. Plus, it has the money to build out a product's functionality, staff requirements and support infrastructure, he added. 

Nelson compares Slack to a growth story like that of data visualization company Tableau (DATA) - Get Report , which saw its market share diminished by similar competing services such as Microsoft's Power BI and Amazon's (AMZN) - Get Report  QuickSight, available via Amazon Web Services. Eventually, its stock took a hit -- Tableau shares are down nearly 50% this year.

Microsoft and Amazon didn't need to spend years creating a full service, just something that's enough to "get the job done," Nelson added. Tableau lost its competitive edge in part because it has a service that can be replicated by other companies, Nelson noted.

"It'sone of the more powerful trends we see in the tech sector, where companies like Microsoft have real staying power," Nelson said. 

For that reason, it doesn't matter much that Microsoft, Facebook and other companies came to market with workplace communication services three years after Slack debuted in 2013, said Frost & Sullivan Principal Analyst Robert Arnold. Most major companies will wait until it seems that there's an attractive entry point in the market to take a stab at it. 

"A critical mass of demand and adoption must be reached before many developers commit the resources needed to develop, market, sell and support such advanced solutions," Arnold said in an email.

Looking ahead, it's unclear whether similar offerings will threaten the possibility of a Slack IPO in the near term. Fushman said the Slack team is "keeping its heads down" as it focuses on growing the product and its customer base.

Rival services are likely to ramp up competition and could even slow Slack's growth, but more importantly, the broad slate of available platforms will generate "a lot of upside" to the market and "help grow the pie overall," Dawson argued. 

Enterprise chat platforms will likely be the next space to adopt artificial intelligence technology. Microsoft is making a "huge investment" in chat bots through its Azure cloud computing platform, Nelson noted. Slack has also had "slack bots" for more than a year now, which can post messages and automate tasks, among other things.

"It's such a nascent market," Nelson said of the workplace communications platform business. "So it's probably too early to tell, but Microsoft, Amazon, Google, Salesforce (CRM) - Get Report will all be leaders in that sphere."