This article mentions "illegal sex activity", but I have no idea what that means. Is it prostitution? Some sort of trafficking deal?
Prostitution is illegal in China and a significant proportion of the female profiles on any dating app are probably created by pimps fishing for customers. That said, enforcement against platforms for facilitation is somewhat random. E.g. the article mentions Tantan being taken down while Momo stays up.
According to this article, a video showing prostitutes receiving training on how to most effectively use Momo went viral a few years ago: