At a recent all-hands meeting with employees, Kevin Systrom, a founder and chief executive of Instagram, showed off one of his favorite charts: Days to Reach the Next 100 Million Users.
"It's the only graph in the company that we celebrate when it declines," Mr. Systrom said in an interview last week at Instagram's headquarters in Menlo Park, Calif.
Not long ago, the Facebook-owned photo-based social network grew at a steady clip. Every nine months, without fail, Instagram added another 100 million users somewhere in the world. Then, last year, it began racking up more new users every day. It grew to 600 million users from 500 million in only six months.
On Wednesday, just four months after reaching that milestone, the company announced it had reached another: About 700 million people now use Instagram every month, with about 400 million of them checking in daily.
I had come to visit Mr. Systrom because I'm one of the new 100 million. I technically joined Instagram years ago but used it only occasionally. In the past few months, however, I began diving in more often, and now I check it several times a day. As I used Instagram more, I realized something about the photo-sharing app: It's becoming Facebook's next Facebook.
Part of what got me interested in using Instagram more was the war between Facebook and Snapchat, the picture-messaging app that has created genuinely new ways of communicating online — and whose features Instagram and Facebook's other subsidiaries recently copied.
But once I started using Instagram, I discovered something surprising: Instagram has improved on the features it took from Snapchat. Over much of the past year it has added lots of other features, too. Among them are a feed ranked by personalization algorithms rather than by chronology, live streaming, the ability to post photo galleries and a (controversial) new app design and logo.
Instagram is now substantially changing the daily experience of using the service at a speed that would ordinarily feel reckless for a network of its size. But rather than alienating existing users, its confident moves seem to be paying off.
This is difficult to quantify. My subjective experience may not match yours (lots of people, for example, say they hate the new ranked feed). But for me, Instagram's many changes have made for a social network that feels more useful, interesting and fun than it was last year. Part of it is the new features themselves, but a bigger reason is the greater use that the features have inspired. Networks are better when more people use them more often; the more I've used Instagram recently, the more stuff I've seen from more people, and the more I want to use it some more.
Instagram has thus triggered an echo — it feels like Facebook. More precisely, it feels the way Facebook did from 2009 to 2012, when it silently crossed over from one of those tech things that some people sometimes did to one of those tech things that everyone you know does every day.
In some ways, this is not surprising. Instagram has been growing like crazy essentially since it went live in 2010, and under Facebook — which bought the company for $1 billion five years ago — it has had ample resources to keep that up. But with 700 million users, it's in virtually uncharted territory.
There are bigger networks: Facebook has nearly two billion users a month, and two instant-messaging apps owned by Facebook, WhatsApp and Facebook Messenger, have grown past the one-billion-user mark. In China, WeChat also has more users.
But last year, you might have said there was a question whether a picture-based service like Instagram could have reached similar scale — whether it was universal enough, whether there were enough people whose phones could handle it, whether it could survive greater competition from newer photo networks like Snapchat. Maybe those problems or others will rear up in the future, and growth could yet stall. But for now, Instagram seems to have overcome any perceived hurdles. It seems to have reached escape velocity.
Mr. Systrom said this plan to rapidly speed up Instagram's pace of change to attract more users was deliberate.
"The primary reason we've scaled more quickly in the last 100 million is that we've figured out that as we've scaled, we've had to unbreak ourselves," he said. What he meant was that Instagram systematically analyzed all the bottlenecks to its service and tried to eliminate them. Then it looked for potential opportunities to better serve users and tried to put them in place as fast as possible.
This sounds trivial — aren't all companies looking to constantly improve? — but social networks are sometimes held hostage by their most loyal users, who tend to hate change (cough, Twitter, cough). Facebook bucked that trend; as it grew, it constantly adapted its features to become more things to more people. Mr. Systrom is following the same playbook.
"My favorite thing to ask the team is, how large do you think Instagram will be eventually?" he said. "Usually you get to some large number, and it's definitely more than two times the size we are now. So I can confidently say that most of the people who'll eventually use Instagram don't use Instagram now."
Mr. Systrom is a fan of academic business theories, especially Clay Christensen's, whose "Innovator's Dilemma" addresses the tension between serving an incumbent audience at the expense of a much greater potential one. The realization that Instagram could become much bigger than it is now was freeing, Mr. Systrom said; it gives the company the confidence to keep changing.
Some of the bottlenecks the company has addressed in the past year are internal. For example, Mr. Systrom and his co-founder, Mike Krieger, realized that one of the primary holdups was their own decision-making. So in the past three months, they started holding meetings in which they just make a bunch of decisions.
"We have a doc in which we list out the inventory of decisions on products — as if it's stacked up in front a machine, waiting to be processed," Mr. Systrom said. "And then we have sessions where we sit down and we decide. You just work through the decisions."
Other bottlenecks involved technical fixes. More than 80 percent of Instagram's users are now outside the United States, and the service is growing especially quickly in parts of Asia and South America that are dogged by underpowered Android phones and slow cellular networks. (Snapchat, for example, has had trouble with Android performance.) A huge part of Instagram's engineering efforts are thus devoted to making its Android app work better outside the United States. For instance, after Instagram began Stories — the video-slide show feature it took from Snapchat — it spent a month adding speed improvements for international markets.
"We consistently find that performance improvements lead to usage improvements at the level of what a new feature would add," Mr. Krieger said.
And then there's Instagram's decision to incorporate features developed by Snapchat, about which Mr. Systrom was unapologetic. He credited Snapchat with creating Stories, but argued that Stories was no mere feature, but instead a brand-new digital format — something like digital feeds (for instance, Facebook's News Feed or Twitter's stream of tweets) — that could be broadly reinterpreted across different products.
"I don't know much about the history of cars, but let's say the Model T was the first car," he said. "So what do you think the first car company other than Ford was thinking? Are we copying Ford, or is this a new mode of transportation that everyone is going to have different takes on?"
This can sound a little too defensive, but it's not exactly wrong. If you compare how Stories works on Instagram with how it works on Snapchat, they are indeed similar. But the context of the two apps — the fact that Instagram tends to foster larger, more public networks in which people maintain a more polished profile, while Snapchat encourages a smaller, more intimate network — does change the nature of the format. Stories on Instagram feels different from Stories on Snapchat because there are different people on both networks using it for different purposes.
And for me, the Instagram version often offers a superior experience for one obvious reason: I know more people there, and you most likely do, too.