Instagram has doubled its user base, to 700 million monthly actives in two years, fueled by Stories, web signups and better onboarding on low-end Android phones. Instagram’s growth rate is actually speeding up. It took just four months to add the last 100 million users since hitting 600 million in December, while it took six months to go from 500 million to 600 million.

Here’s a breakdown of how long it took Instagram to add each 100 million users:

October 6, 2010 – Launch

February 26, 2013 – 100 million; 28 months

March 25, 2014 – 200 million; 13 months

December 10, 2014 – 300 million; 9 months

September 22, 2015 – 400 million; 9 months

June 21, 2016 – 500 million; 9 months

December 15, 2016 – 600 million; 6 months

April 26, 2017 – 700 million; 4 months

For reference, this makes Instagram more than twice the size of Twitter. Soon it might join Facebook’s other properties in the billion-user club, since WhatsApp and Messenger have 1.2 billion users and Facebook 1.8 billion.

When asked if the launch of Snapchat clone Instagram Stories helped boost the app’s user count, a spokesperson told me, “Yes, it’s impacted growth and retention.” Instagram Stories reached 200 million daily active users earlier this month, eclipsing the 161 million total daily users Snap Inc. last announced. Instagram’s combined ephemeral and permanent messaging feature Direct, also inspired by Snapchat, recently hit 375 million monthly users.

Instagram’s progress has come at the expense of Snapchat’s growth, which sank 82 percent since Instagram launched its Stories copycat in August. All eyes will be on Snap’s user count when it gives its first earnings report next month.

Meanwhile, as Snap deprioritized foreign markets with lower ad rates, Instagram has continued to push its international presence. “Developing markets have always been important at Instagram,” says a spokesperson. “U.S. is our biggest market, but Brazil has long been our No. 2 (currently with 45 million users). Along with the U.K. and Japan, our other big markets include Indonesia, India and Turkey.” Now 80 percent of Instagram users are outside the U.S.

Instagram has been building products specifically for these users who can’t afford data plans or have weak connectivity. It just introduced offline mode on Android, and the company tells me it’s been focusing more on its mobile web experience as well as keeping the Android app’s file size to a minimum.

“Growth was driven by a number of factors, including our ability to better connect people with their friends on Instagram,” said a spokesperson. “We also took steps to simplify and improve our sign-up process, which helped bring in new people from all over the world.”

With such a large audience, Instagram has the scale to appeal to top brands. The app now has 1 million active advertisers, up from 500,000 in September, with plans to offer them more data and flexibility.

It’s rare for a social network to stay as cool after 6.5 years. Even its owner Facebook was starting to look a bit old at that point. But by sticking to its roots of visual communication while shrewdly adapting to new trends, Instagram has managed to stay on top.