I actually ended a friendship with a childhood friend a few years back over something like this. He refused to ever come visit me in China (from France) as he was persuaded he'd get arrested for having posted negative things about China on social media, and couldn't be convinced otherwise. I actually checked the data: how many foreigners have ever been arrested, or detained, in China, for writing negative things about the country on social media? The answer is zero, it never happened, not a single case. Which means the risk that this happens is virtually non-existent. China couldn't care less what you write about them on Twitter or Facebook. In fact I suspect they'd be especially pleased if someone who wrote negative things were to visit China, as it shows some degree of openness to change their mind. That's partially why, incidentally, China now has a visa-free policy with so many countries (including almost all European countries): they know full well this will mean that a lot of people with negative preconceptions will visit. Is the idea to detain them all? That'd be rabidly idiotic, as it'd just confirm their biases and dissuade anyone to visit China. The idea, on the contrary, is that they see with their own eyes that the country is much nicer from what they believed it to be.