TikTok was not available for many of its 170 million users in the U.S. hours before a ban on the social media platform was supposed to go into effect.

Challenges came in tandem with TikTok’s success. U.S. officials expressed concerns about the company’s roots and ownership, pointing to laws in China that require Chinese companies to hand over data requested by the government. Another concern became the proprietary algorithm that populates what users see on the app.