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.