Instagram can become bigger than Facebook.

Instagram, the popular photo-sharing app, hit 100 million users earlier this year, triple its user base since Facebook acquired the startup for $1 billion in 2012. With such unprecedented growth, will Instagram become bigger than Facebook?

"By definition, if it keeps growing at this rate, yes, it will be bigger," Instagram cofounder and CEO Kevin Systrom told Fast Company. "It will be the biggest thing in the world."

Systrom shared the revealing comments on our recent trip to Instagram's office, as part of Fast Company's feature on the company's first year at Facebook. Systrom's words serve not only as a sign of his own strong belief in his blossoming social network but also as a larger symbol of the potential for Instagram inside its parent company. Facebook, with roughly 1.1 billion users, is the world's largest social network, but there is concern that its popularity is waning, especially among younger users, who are migrating to up-and-coming services like Snapchat. But with Instagram still breaking out from under its yoke, Facebook can be sure it made the right bet on the startup. After all, Systrom isn't resting on his laurels. "Now, will we continue to grow at this rate [and outgrow Facebook]? Different question," he says.

Perhaps it's irrelevant whether Instagram passes Facebook in popularity. (Is Disney upset that Pixar's Monsters University crushed Disney's The Lone Ranger at the box office? Well, maybe.) What's important is that Instagram is keeping a new generation of users interested in Facebook products--while stealing engagement from its competitors. Reports indicate that Instagram has topped Twitter in mobile engagement. And more recently, Instagram's video service is said to be growing fast in popularity, though reports vary whether it is outpacing Vine, Twitter's video-sharing service.

Inside the company, as we outlined in our feature on the two companies, the cross-pollination between Instagram and Facebook indicates the two are taking a rising-tide-lifts-all-boats approach to product. As Instagram cofounder Mike Krieger put it, perhaps more diplomatically than Systrom when asked whether Instagram will outgrow Facebook: "I don’t know--that's a good question. I think both of them are basically limitless in size, so maybe one day they’ll be the same size?"