Instagram is now bigger than Twitter after the number of monthly users increased by 100 million in just nine months.

The photo-sharing app - bought by Facebook for US$1bn in 2012 - is growing faster than analysts expected with 400 million regular users.

Rapid growth in countries such as Brazil and Japan is one of the main reasons for the soaring user numbers.

The social network is also playing an increasingly important role for brands, including Disney which posts movie trailers to the service.

By comparison Twitter - which competes for the same advertisers as Instagram - reported 304 million monthly active users in its most recent quarter, up just 12% year-on-year.

Instagram is currently trying to expand its advertising operations, offering brands 30-second video slots and Twitter campaign-style "marquee" adverts.

Adverts first started appearing on Instagram two years ago.

Facebook has not released Instagram's revenue figures, but one analyst estimates them at US$600m for 2015.

No Facebook branding appears on Instagram, which has a degree of independence from the world's biggest social network.