After nearly doubling its US user base last year, growth for TikTok will slow in the coming years as competition heats up and concerns grow among marketers.

TikTok’s US user base will grow 21.9% this year to 45.4 million people. By 2021, it will surpass 50 million (52.2 million). This follows 97.5% growth in 2019, as the dynamic, short-form video platform drew in a huge number of users, especially children and teens. Growth will slow to single digits in 2022 as the app becomes heavily saturated among core younger users.

“TikTok had a breakout year in 2019, and it is incredibly popular among teens at this point,” said eMarketer principal analyst Debra Aho Williamson. “Some are spending multiple hours per day on the app, which is a testament to the incredible stickiness of its scrolling video format. But it has yet to develop a strong following among older generations.”

This year, 21.6% of US social network users, or more than one in five, will use TikTok at least once a month. Despite having lower penetration than more established competitors, it’s one of the few social apps whose penetration is growing.

Competition may also hinder growth for TikTok in the coming years.

“This year, we expect new entrants like Firework and legacy players like Instagram and Facebook to take on TikTok,” Williamson said. “Instagram is a particularly large threat; it recently added new tools to its Boomerang looping-effect feature that mimic similar features on TikTok.”

And while many marketers are curious about the app, several issues could hold back adoption. Because it is Chinese-owned, TikTok faces questions about censorship and data privacy, as well as brand safety.

“Properties like TikTok are supporting and creating new forms of self-expression, and brands want to tap into that,” Williamson said. “It’s an experiment for many marketers. In many ways, TikTok is where Facebook was in the late 2000s, and where Snapchat was three or four years ago.”