Not entirely sure, I’ve never had to decouple someone from social media. My relatives are hopelessly addicted to Facebook.

A suggestion would be to try to gently persuade them of the harms of WeChat, but not to be too pushy about it, could cause them to push back or use it on secret. If they care about their privacy, perhaps taking that route would be beneficial. Having some articles on hand that affirm your argument can help too.