ZAO, a new Chinese app that lets users switch faces with celebrities, sports stars, or anyone else in a video, has gained millions of downloads, but the happiness of users with the prospect of becoming stars quickly became strained when the repercussions of privacy began to emerge.

The application “ZAO” has sparked criticism about privacy issues, highlighted the increasing popularity of such applications by some users, and how artificial intelligence techniques raise new concerns surrounding identity verification.

The ZAO app was launched recently, and currently tops the list of free apps on the iOS store in China, and its popularity helped bring another face-changing app called Yanji to fifth place on the list.

According to a post from app makers on the Chinese Weibo platform similar to Twitter in China, ZAO’s servers have almost been down due to increased traffic.

The ZAO application was developed by a wholly owned subsidiary of Momo Inc., and consumers subscribed to ZAO via phone number, uploading photos of their faces taken with their smartphones.

They can then choose from a range of celebrity videos, switching faces to celebrities, and sharing videos with friends. An earlier version of the user agreement for the application stated that consumers who carry their images to ZAO agree to hand over their intellectual property rights to their faces, while allowing the application to use their images for marketing purposes.

The amended version of the user agreement states that the company will not use face images or videos uploaded by users for purposes other than improving the app or things that users have previously approved.

If users delete the content they have uploaded, the app erases it from their servers as well. “We understand privacy concerns, and we will work to resolve issues that we didn’t consider, but we need some time,” the company said.