As Snapchat parent Snap Inc. prepares to go public this week, it will have a job on its hands to fight off Instagram.
According to a study by eMarketer, Instagram will have 77 million U.S. users in 2017, while Snapchat will have 70.4 million.
What's more, that divide is likely to widen by 2021, when Instagram will have 98.7 million U.S. users, compared with Snapchat's 89.2 million, according to the study.
Meanwhile, WhatsApp — owned by Facebook — has also launched a rival to Snapchat, as did Instagram, which is also owned by Facebook. Its WhatsApp Status lets people share photos and videos in a montage that disappears after 24 hours.
While Snapchat is often thought of as a teenagers' platform, people over 45 will drive some of its growth, according to eMarketer, which estimates that 6.4 percent of U.S. Snapchat users will be between ages 45 and 54 this year, up from the 4.2 percent it previously forecast.
The projection from eMarketer includes people who access their Snapchat app at least once a month.
Follow CNBC International on and Facebook.