First, it's impressive growth. Since March, the number of new monthly active users has totaled more than 100 million.
Second, this now makes the photo-sharing app, which Facebook bought for $1 billion in April 2012, bigger than Twitter, which has 284 million monthly active users, and whose market cap is around $23 billion.
More than 70 percent of its users are outside the U.S., and users have shared more than 30 billion photos.
"We're seeing a lot of people coming in the fashion world, a lot of people coming in, in the youthful teens world, and a lot of people internationally as well," CEO Kevin Systrom said in an exclusive interview with CNBC.
Can Instagram keep up that pace of growth? "That's the big challenge for me and my team going forward," he said.
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One way Systrom aims to do that is to make it easier for people to discover content and find people to follow. "There are photos pouring in from everywhere in the world. If you're interested in really eclectic topics, I guarantee you there are accounts that are related to that. So we need to make sure to connect you with the content that matters most to you."
The 300 million marker isn't Instagram's only announcement Wednesday. As part of an effort to eliminate spam, it's announcing verified badges for celebrities, athletes and brands to help people ensure they're connecting with authentic accounts. The company says it's also working to deactivate spammy accounts to improve the user experience