Venture firm Accel is known for making one of the most lucrative start-up bets in history, parlaying an early $12.2 million investment in Facebook into a stake worth more than $9 billion at the time of the company's IPO in 2012. Seven years later, Accel is enjoying a comparable stretch of success. The firm is the biggest investor in Slack, owning a 23.8% stake as the messaging app hit the public market on Thursday. Based on an opening price of $38.50 on the New York Stock Exchange, Accel's shares are worth $4.6 billion. That's a decade after the firm put $1.5 million into the seed round of a gaming company called Tiny Speck, which later morphed into Slack.

Norma Butterfield, mother of Slack Technologies CEO Stewart Butterfield, rings a ceremonial bell along with New York Stock Exchange (NYSE) President Stacey Cunningham during the Slack direct listing in New York, U.S. June 20, 2019. Brendan McDermid | Reuters

"As investors it is easy to get lost in the complexities of product market fit or connecting a young company to an established category," Accel's Andrew Braccia wrote in a blog post Thursday. "Slack is a great reminder that the most interesting companies are often the ones whose courses aren't easily charted. The commonalities between those companies start and end with the drive, creativity and resilience of their founders and early team members." Braccia, who's a Slack board member, made similar comments in a 2015 interview with TechCrunch. He said he talked to Slack co-founder Stewart Butterfield at the time of the pivot about whether the company should return the money now that the gaming idea was clearly a flop. "But the company we'd invested in was Tiny Speck, which produced the game," Braccia told TechCrunch. "And the reason we invested in Tiny Speck was because we were investing in that team. I told Stewart, 'If you want to continue to be an entrepreneur and build something, then I'm with you.'"

Butterfield is a billionaire