Brief:

• Instagram's global user base is predicted to increase 56% to 927.9 million people by 2021 from 593.7 million this year, eMarketer estimated in its first forecast for the image-sharing app owned by Facebook. While Instagram claims to have 800 million users, eMarketer's estimate made adjustments for duplicate accounts, business profiles and fake users.
• Instagram is most popular in its home country with 85.5 million U.S. users, but eMarketer estimates that international markets will be a bigger source of audience growth.
• Meanwhile, worldwide ad revenue on the platform will more than double from $4.1 billion this year to $10.87 billion by 2019, per the forecast.

Insight:

While eMarketer estimates that Instagram's audience will approach 1 billion users in the next few years, the app's rapid growth rate doesn't appear to be sustainable, as it's expected to decline to 7.4% by 2021 when it reaches a saturation point among smartphone users. That signals that the app's sales growth will likely become more dependent on developing additional services to keep users engaged on the platform for longer periods.

The forecast attributes much of Instagram's growth in the past two years to adding features such as live video, location-based stickers, face filters and Stories, which string together several images and videos into a single post. Snapchat introduced many of these features first, but they've also made the app more dependent on broadband connectivity, eMarketer said. Snapchat may need to introduce a "lighter" version to appeal to audiences in emerging markets to spur growth.

Instagram is especially popular among younger consumers in emerging markets who don't have access to broadband wireless connections. That's a key difference from rival Snapchat that has thus far focused on mature markets and high-end mobile devices.

Meanwhile, Snapchat is the most popular social media platform among U.S. teenagers, according to a recent study by RBC Capital. The bank found that 79% of 13- to 18-year-olds have a Snapchat account, compared to 73% for Instagram and 57% for Facebook. The photo-messaging app is the preferred social media platform for nearly half (47%) of teens, marking an increase of 12 percentage points year-over-year, Piper Jaffray found in an October survey. Just 24% of teens said Instagram was their favorite social media platform, marking a rise of 1 percentage point, while both Facebook and Twitter dropped in popularity to 9% and 7%, respectively.