Instagram, which launched in 2010, edged past 8-year-old Twitter and its reported 284 million monthly active users.
Facebook, which bought Instagram in 2012 for $1 billion, has nothing to worry about. In September, the social network reported that its own active monthly user base had hit 1.35 billion.
"Over the past four years, what began as two friends with a dream has grown into a global community that shares more than 70 million photos and videos each day," wrote Instagram CEO Kevin Systrom in a blog post. "We're thrilled to watch this community thrive and witness the amazing connections people make over shared passions and journeys."
Zeus Kerravala, an analyst with ZK Research, said Instagram's impressive growth stems from its popularity with millennials, who have a strong connection with social networking, selfies and images.
"The younger generation, in particular, is a very visually oriented group that loves selfies," Kerravala said. "Pictures just say more -- they're fast and easy. Instead of saying, 'What a great view of the Grand Canyon,' snap a photo and upload it."
He noted that Twitter users can upload photos and short videos to the micro-blogging site, but the site is mainly used for its 140-character or less messages.
"I think Twitter is more for information dissemination, where Instagram is more about sharing content," Kerravala said.
In his blog post, Systrom said that while the site is becoming more popular, he wants to make sure that no one is impersonating a celebrity.
"People ask, 'When it comes to public figures and brands, how do I know that the people I discover on Instagram are really who they say they are How do I know that this person really is Tony Hawk'" he said.
To address that issue, the site on Wednesday launched verified badges for celebrities, athletes and brands in an effort to help assure users that the people they're following are who they say they are.
The badges will start rolling out this week.