Instagram CEO Kevin Systrom had a key role to fill in early 2016. He was seeking to revamp the photo-sharing service he’d co-founded with Mike Krieger six years earlier, and which Facebook had bought in 2012.

After keeping Instagram’s executive ranks intentionally small, Systrom was putting together a team to help him overhaul the site.

The objective was to get users—especially younger ones—to post more content on Instagram.

At the time, Snapchat was growing fast among youthful social media users, thanks to fun features like funny photo filters and disappearing videos.

Systrom had already poached some big names from Silicon Valley rivals, getting Ian Spalter from YouTube as head of design in early 2015 and hiring Yahoo’s James Everingham as head of engineering six months later.

Now he needed someone to get the features and tools being designed and built onto the site and into the hands of Instagram users.

As he searched for someone to help him implement his product roadmap, Systrom asked a friend, venture capitalist Elizabeth Weil, to arrange a dinner.

Weil, who’d spent four years as a partner at the marquee investment firm Andreessen Horowitz before becoming managing partner of a new firm, 137 Ventures, is married to Kevin Weil, whom she’d met when they were both students at Stanford University.

Kevin Weil had quit the university’s Ph.D. program in theoretical physics to start working at Twitter in 2009, when the social media service had just 40 employees.

After seven years at Twitter, he had worked his way up to executive vice president overseeing all of the company’s products, including its consumer-facing site, advertising products and the Vine and Periscope video tools.

Elizabeth Weil arranged the dinner.

During it, Systrom offered Kevin Weil a job running product for Instagram, according to an on-stage interview Weil gave in May to Stanford professor Tina Seelig, as part of the university’s discussion program for prospective entrepreneurs.

Weil says he turned Systrom down. He’d already resigned from Twitter, where user growth had begun to wane, and planned to take six months off to train for a 50-mile foot race that traveled the course of the American River in California’s Central Valley. What’s more, he and his wife had started a family.

Weil, who is 34, was enjoying time off and had just attended the Super Bowl, which was held that year in Santa Clara, California, according to his Twitter feed.

But then he reconsidered.

“I said ‘no,’ then said to myself: Why should I turn this down?” Weil said in the video.

Weil, a whiz at math and later at physics, grew up in Redmond, Washington, home of Microsoft, where his dad had worked for two decades. After his own long stint with one company, Weil told Seelig, “I wanted to see how other companies did things.”

Weil took the job in March 2016—and then finished the American River 50 in April, where he placed fifth.

“I like running long races,” Weil stated, a comment that should put fear into the hearts of Snap investors. “There’s not a lot of tricks to running a 50-mile race, you just start running and keep going. People underestimate the value of fighting through it,” he said.

Equally as impressive is what Weil, and the rest of the team Systrom put together, have managed to pull off in the last year and a half.

“He was a great hire…a key player,” said Krishna Subramanian, CEO of Captiv8, a startup that provides analytics to social media sites and their users who create popular content.

“He’s been a critical part of their success,” said Subramanian, who’s sold several companies he’s co-founded previously, including Mobclix and BlueLithium.

Within months of Weil’s arrival, the site began to roll out a raft of new tools and features to match what Snapchat was doing. The key was Instagram Stories, a feature that let users share photos and videos for up to 24 hours before they disappeared, mimicking the similar Snapchat Stories feature.

In November, it began letting users post images and videos that disappeared right away—the feature that originally made Snapchat famous—and in May 2017 it added face filters, one of Snapchat’s most popular features.

Soon Instagram’s growth in both users and ads began to surge.

It was aided not only by the tools Weil and others helped get out the door but also by a savvy digital marketing plan: Facebook sent repeated notifications to its own users telling them which of their online friends were already on Instagram.

The plan worked.

In June 2016, Instagram had 500 million monthly users. By April 2017, it had grown 40 percent to 700 million.

More users brought more online marketers. In late March, Instagram hit 1 million advertisers, double the 500,000 it had last September.

As its growth accelerated, Snapchat’s began to flatten out.

In the first quarter, Snapchats number of daily users grew only 36 percent from the previous year, to 166 million. Instagram Stories, meanwhile, has grown to 250 million daily users in less than a year on the market.

“It’s the fastest-growing product I’ve ever seen,” Weil stated in the video.

Instagram, which Zuckerberg bought for $1 billion five years ago, is expected to post sales of $3.6 billion in 2017, according to the market research firm eMarketer.

When Weil joined in spring 2016, he said in the the Instagram brass had come to a conclusion.

“A year ago, Instagram was a product for your highlights,” rather than a place where users posted content about more routine events in their lives, Weil said in the video.

In other words, it had not changed much since Systrom had started it as a sort of daily photo diary back in 2010.

“If people are going to post multiple times throughout the day, the product needs to be more ‘person-oriented’ than ‘feed-oriented,” Weil explained.

That meant allowing users to post not only square photos but videos of varying shapes, sizes and lengths.

Instagram also has a culture designed for doing “the simple things first.”

In the video, he says, “If you start simple, systems will get more complex (eventually)…If you start complex, you’re in trouble.”

Also, while big product launches are important, “what people miss are the little changes…Facebook and Instagram are better at that than anyone,” he says.

Weil, whose 30-person team (as of May) work “all in the same room,” also credits Systrom and Zuckerberg for maintaining Instagram’s independence.

“I give a lot of credit to the two founders..We’re a tiny company that gets to be a superpower because we’re part of Facebook,” according to Weil.

As for Snapchat, Weil gives it its due— up to a point.

“Snapchat was the first to create the format…more power to them, they did a fantastic job,” he said.

In the 15 months since Weil joined Instagram, something else has grown:

His wife had twins and now the couple have three very young children, according to his Twitter feed and the video.

When Seelig asked whether his life has “balance,” Weil chuckled and described his home life as “insanity…life is insane between six and nine (p.m.)”

Asked whether he sleeps, Weil answered “not much, that’s mostly what we sacrifice.”

In spite of all that, this past spring he ran the American River 50 again and did slightly better, finishing fourth.

In a fitting bit of irony, Weil also attended Harvard at the same time as Mark Zuckerberg, who never finished his Harvard degree. (Zuckerberg received an honorary degree last month, when he delivered the university’s commencement speech.)

After Zuckerberg left for California and never came back, Weil, who as a junior was one year ahead of Zuckerberg, described his reaction.

“I’m sitting there going, ‘He’s giving up a Harvard degree to go work on some company? What an idiot!’…Now 12 years later I’m working for him.”

Here’s the video: