Instagram announced this week that it has more than 500 million users, and about 300 million of them use the photo- and video-sharing app every single day. The vast majority – over 80% -- live outside of the United States, according to the company.

CEO Kevin Systrom told Forbes it’s a milestone worth celebrating, but that the five-and-a-half-year-old company has no plans to rest on its laurels. “This scale is not a badge on our uniform, but a signal of our ambition. If we can have a billion or a billion and a half on Instagram, we get closer to capturing every experience in the world,” he said.

Back in 2012, when Facebook purchased Instagram for $1 billion, the app had fewer than 22 million monthly active users, but it has expanded rapidly since then and is, in fact, considered one of the fastest-growing apps of all time. Instagram’s advertising business has also been growing. The app is expected to generate $1.5 billion in global advertising revenue this year, according to a forecast by eMarketer. By 2018, that revenue could be as much as $5.1 billion globally. A recent Facebook study determined that 60% of Instagram users have learned about products and services on Instagram, and 75% have taken action based on what they see on the app.

The secret to Instagram’s success, according to Systrom, is the fact that users can follow someone without having to ask first. “There is a whole world of behavior around letting everyone see your photos,” he told Forbes. “I don’t have to request you as a friend to see your photos. That’s Instagram’s super power.”

This milestone is the latest in a series of big announcements from the company. Last month, Instagram unveiled an updated icon – a stylized, simplified iteration of a camera with a rainbow-like background. Earlier this month, Instagram announced it would be launching new tools for businesses, including special profiles, data analytics and the ability to turn posts into ads.