$3.8 billion Slack's chief marketing officer quietly stepped down after less than two years

Slack's head of marketing has quietly stepped down after two years on the job, leaving a key position at the $3.8 billion startup unfilled just as it faces tough new competitors.

Bill Maciatis, a tech industry veteran of who was hired as Slack's first Chief Marketing Officer back in 2014, stepped down a few months ago and move to an advisory role at the compay, according to his LinkedIn page.

A Slack representative confirmed to Business Insider that the company has not yet replaced Maciatis.

"Bill continues to be closely engaged with Slack, and is also following a passion of his to mentor other high growth startups by serving as formal board members and advisor roles," says a Slack spokesperson. Maciatis' LinkedIn bio, which notes that he had also served Chief Revenue Officer, says he remains on Slack's board of directors.

The change comes at a time when Slack is battling to new big-name competitors, with Facebook's new Workplace service and Microsoft's Team products both aiming to scoop up the business users that have flocked to Slack's messaging platform.

The lack of an experienced marketing leader could prove tricky for Slack as it looks to set itself apart from the competition. On Wednesday Slack published a full-page advertisement in The New York Times "welcoming" its new rival Microsoft to the enterprise chat market. The ad got plenty of attention, but was also criticized for making the startup appear overly defensive and not in keeping with Slack's friendly brand image, embodied by founder Stewart Butterfield.

Slack, for its part, has been redoubling its efforts to sell to larger customers: Back in May, Slack hired Salesforce veteran Robert Frati as its first-ever head of sales, signaling that it was taking the enterprise market more seriously.

And, in general, Slack says that it's growing like crazy, with 650 employees and counting across its four global offices - 15% of whom joined in the last 45 days. Slack also says that it's expecting another 100 people to join in the very short term.

On the product side, though, questions still remain. Big customers like Uber have passed over Slack in favor of rivals like Atlassian HipChat, over concerns that the startup can't keep up with the demands of larger enterprises with thousands of employees. Noted pundit Ben Thompson today wrote he's still optimistic about Slack's chances, but is waiting for the startup to implement long-promised security controls for large enterprises.

For its part, Slack now says that Uber was only ever testing its product with a small group of its then-6,000 employees, and it just never went past that experimentation stage. In general, Slack says, it's dealing with customers larger than Uber was at that time, every day.

"Slack also has customers like IBM with more than 30,000 daily active users who rely on Slack to get their jobs done. That's 5x the total size of Uber at the time they evaluated Slack and at a company with 6,300% more employees than Uber did," says a Slack spokesperson.

Still, Microsoft, at least, doesn't seem too concerned, recently referring to Slack as an "application du jour" for a "very narrow niche." Slack has its work cut out for it to prove the naysayers wrong.