TikTok, the explosively popular Chinese social media platform, may be approaching the upper limits of its global popularity. The platform launched in 2016 as a lip-syncing app called Musical.ly before being acquired by ByteDance, now the world’s most valuable startup. Following an international launch in 2017 and an official rebranding, TikTok has since expanded into a unique hybrid somewhere between Vine and YouTube that’s captured the cultural zeitgeist among teenage smartphone owners in unprecedented fashion.

In the two and a half years since it launched outside China, TikTok has seen astounding growth, adding more than 500 million users so far this year and closing in on 1.5 billion users in total. But according to new data from mobile analytics firm SensorTower, TikTok just experienced its first ever growth slowdown on a quarterly basis.

TikTok’s takeover of social media looks like it’s slowing down

The number of first-time downloads in the quarter that ended September 30th came in at 177 million, a four percent decline from this time a year ago. That drop in user growth may be related to a pullback in Facebook ad spending, Bloomberg reports. TikTok has gained a bulk of its new users in the US by advertising itself using app install ads on Facebook, but it’s not spending as much as it once did. The growth of its US user base has slowed dramatically as a result.

While TikTok remains a global phenomenon, slowing user growth on top of mounting challenges from lawmakers and criticism from competitors, including Facebook CEO Mark Zuckerberg, may prove to be roadblocks in ByteDance’s continued takeover of the social media landscape.

US politicians have been sharply critical of the company over concerns with how it handles user data, as well as for its alleged censorship of political content related to the ongoing Hong Kong protests and other issues deemed sensitive by the Chinese government. (TikTok denies that it censors content on behalf of Beijing.) The company now faces a US national security review, and just today it declined an offer to testify as part of a Senate Judiciary hearing on the tech industry’s ties to China.