Not surprisingly, the company's version of events differs. A spokeswoman told Bloomberg that Instagram "was not pressured" into adding Stories and created it "internally." Co-founder Kevin Systrom painted things slightly differently: he didn't confirm the story, but did say there was "tension" over Instagram's future that ultimately worked out in its favor.
If Zuckerberg did prompt the fateful decision, it wouldn't be surprising as to why. Snapchat was (and to a degree, still is) the go-to service for teens, with usage jumping rapidly in 2015 and 2016. While Instagram was comfortably ahead at the time, Snapchat's pace posed enough of a threat that you could see it eventually claiming the lead. Flash forward to 2017, a year after Instagram Stories arrived, and it was a different story -- Snapchat's growth had tapered off, while Instagram was surging.
And crucially, Instagram's success is crucial to Facebook's success. While it may have far fewer users, it's effectively the alternative social network for people who either object to Facebook (whether due to privacy concerns or its 'uncool' status) or just prefer its photo-centric approach. If Instagram had slipped behind, Facebook critics would likely have left the company's products altogether.