Instagram said in September that it had 800 million monthly active users, but research firm eMarketer has published a report suggesting that the true number is significantly lower.

The company estimates that 593.7 million people globally will use Instagram at least once a month this year: 206.3 million less than the official figure.

Why the disparity? "eMarketer’s forecasting figures take into consideration duplicate accounts, business profiles and fake users," explained the company.

This is the first time eMarketer has published worldwide estimates for Instagram's reach. It claims that Instagram's monthly active user base has grown by 38.7% this year, with the US accounting for 14.4% of its user base – around 85.5 million people.

Its report also includes predictions for Instagram's future growth. eMarketer thinks that in 2018, the app will grow by 17.7% to 698.7 million active users globally, while by 2021 it expects Instagram to have 927.9 million active users.

"The company’s strategic push toward international markets – in particular Southeast Asia – will continue to fuel growth in the years to come," said forecasting analyst Cindy Liu.

eMarketer is also predicting that Instagram's advertising revenues will grow from $4.1bn in 2017 to $10.87bn in 2019 – figures that do not include the money being spent by brands with influencers on the platform.