Insta-Surge! Instagram Hits 400 Million Users, Soars Past Twitter

— -- It's an Insta-surge!

Instagram has attracted 100 million new users in the past nine months, bringing the photo sharing app's user base to 400 million people and putting it well ahead of Twitter's 316 million monthly active users.

More than half of the last 100 million people to sign up for Instagram live in Europe and Asia, according to a company blog post. Instagram continues to expand its global reach, providing even more appeal as Facebook finds ways to monetize the app it purchased in 2012 for $1 billion.

"It is another major milestone that speaks to the age of the visual web where you are starting to see more of a shift from a content perspective to pictures. That whole dynamic has played into Instagram's strengths," Daniel Ives, managing director of the technology, media and telecom research group at FBR Capital Markets, told ABC News.

With more than 80 million photos shared through the app each day -- some from newly joined celebrities including David Beckham and Caitlyn Jenner -- Instagram has began experimenting with sponsored posts in the app.

"There is a battle royale for mobile ad dollars, and that is what the investors are trying to figure out. Who is best positioned?" Ives said. "Twitter has definitely hit some speed bumps over the past year."

Two of the biggest tasks facing Twitter interim CEO Jack Dorsey are adding new users and retaining current ones.

"The opportunity is absolutely massive. The intention is to certainly bring it to everyone around the world," Dorsey said in a conference call with investors in June. "If we build a product people love and value, advertisers and users will follow."

Dorsey, who co-founded Twitter, took over as interim CEO in July as the company's board continues to search for a permanent replacement to lead the company.