On the face of it, there’s nothing surprising about Instagram’s founders leaving six years after the company was sold. Mike Krieger and Kevin Systrom’s tenures at Facebook were longer than that of most Facebook employees, where the average is 2.5 years. And Instagram has come a long way since Facebook bought it in April 2012 for a reported $1 billion. In the past six years, Instagram has grown from 50 million users to more than a billion users, and it currently employs more than 700 people. Today, its estimated worth is over $100 billion.

When Facebook acquired Instagram, it promised that it would not mess with the company. But the truth may be that Instagram has become far too important to Facebook’s bottom line for Mark Zuckerberg to keep that promise. With Mr. Krieger and Mr. Systrom’s departures, the future of Instagram is now completely in Mr. Zuckerberg’s hands.

Instagram’s explosive growth is a success story in its own right, and a big part of the credit is due to Instagram’s executive team, who carried out their vision of an uncluttered feed of photographs. Another part of the company’s story, however, is how much Instagram was able to leverage the technical and advertising infrastructure built by its parent company.

Building a platform that can handle millions of users who are uploading millions of photographs every single day is an extraordinarily difficult task, and one that Facebook had mastered. Even companies like Snapchat and Twitter (which reportedly also tried to acquire Instagram) have struggled with performance issues, both with their back-end software infrastructure and their mobile applications. It’s easy to imagine an alternative timeline, one in which Instagram struggled to cope with the scaling challenges, unable to improve its platform, perhaps even collapsing under its own weight.

The widely reported news is that Instagram’s founders grew tired of their parent company’s constant interventions in their product. Some of these interventions, like removing attributions to Instagram photos that appeared on Facebook’s news feed, seemed intended to benefit Facebook at the expense of Instagram. But others, like Instagram’s Stories feature (which was admittedly copied from Snapchat and initially resisted by Instagram’s team), not only kneecapped the growth of Snapchat but also injected a new wave of growth to Instagram itself. Mr. Zuckerberg, for all his foibles, knows a viral product when he sees one.

It isn’t enough for Instagram to merely be big. Just like any other publicly listed tech company, growth is the main driver of valuation at Facebook. And it is both the growth potential of Instagram itself and what it can do for Facebook’s top line that makes it far too important to be left alone any longer. Instagram’s one billion monthly users is an eye-popping number, but it’s less than half of Facebook’s, representing a big potential upside. Moreover, while Facebook users are getting older and becoming less attractive to many big advertising spenders, Instagram has the attention of a much younger generation. To top it all off, Facebook has been battered by scandal after scandal, especially following the 2016 elections, while Instagram has stayed above the fray and out of the spotlight. A chance to redeem itself in the eyes of its users could be too hard to resist for Facebook.

Until now, Instagram focused on keeping its product simple. Instagram is the primary social network of its younger users, while Facebook acts as more of a directory and a conduit to Facebook’s Messenger app. But Facebook still commands higher premiums for the ads it shows, paving the way for Facebook to build more offramps from Instagram into the main Facebook product. Such tricks might boost Facebook’s ad revenue, but they also run the risk of upsetting users on Instagram, especially those who don’t even know that Facebook owns their favorite app.

What Mr. Zuckerberg is planning for Instagram’s future is anyone’s guess, but there are hints here and there. One possibility is that Facebook itself will become more like Instagram. The Stories feature is a good example of how this might unfold for other features in the future: The introduction of the format has been so transformative for the Facebook ecosystem that the company expects time spent on it to surpass time spent on News Feed. WhatsApp stories themselves already have more users than Snapchat. As Ben Thompson of Stratechery shrewdly pointed out, this represents an opportunity for Facebook to delve into new advertising territory but also risks alienating current advertisers.

Another option is that Facebook will turn Instagram into its own media hub, slowly moving away from a pure social network into one that’s more amenable to advertisers. While Facebook has decidedly won the social networking battle against Google, Mr. Zuckerberg and team were never able to crack the code on video like Google did with YouTube. Facebook’s own video products have always been a mixed bag: its broadcasting platform has been plagued by live broadcasts of actual murders and its Watch product hasn’t received much traction. Instagram represents a few more plays, with its focus on visual imagery, over Facebook’s text heavy format. The first few attempts, including the IGTV launch, don’t inspire much confidence, but Mr. Zuckerberg isn’t known for giving up easily.

There is a third option, in which Instagram itself turns into Facebook. Just a few days before the founders’ departure, news emerged that Instagram might be launching a “regram” feature, akin to retweeting on Twitter. Interestingly enough, Instagram long avoided building such a feature, because the founders feared it would make the product less personal. Such a feature could lead users to feel that their feed no longer contains things they wanted to see, but what others pushed on to them, effectively turning Instagram into Facebook.

The third option seems most likely to me: a slow and steady transformation of Instagram into another growth funnel for the Facebook behemoth. Putting myself into Mr. Zuckerberg’s shoes, it’s hard to resist the temptation. Instagram would not be where it is without Facebook’s technical, product, and advertising prowess.

Still, one question remains: What would have happened to Instagram if Mark Zuckerberg had kept his promise, and Facebook had really left it alone? Would Instagram’s founders been able to fund and scale their huge operation, and eventually deliver the giant returns that investors expected? Mr. Zuckerberg certainly didn’t seem to think so, which is why he has finally brought the product under his control. That’s a shame. I haven’t used Facebook in years, but I really liked Instagram.

Can Duruk is a former software engineer. He is earning an M.B.A. at Insead in Singapore.

© 2018 The New York Times Company