I have lots of these girls on wechat and kakao. never heard someone say its friends and family only, but i do know girls that run multiple copies, which I do also to keep work people separate from providers. I keep most of my friends on the same one I use for providers, which keeps my work one very tight.
Yes the chinese government does have access to wechat conversations/logs. They monitor it mostly for financial transactions (wechat is a payment system kind of like paypal and applepay combined, and 100 times bigger), and political conversations. That second part isnt significantly different the what the US government does.
The company that owns wechat, tencent is no closer to the chinese government than apple or google is to the us government. I guess thats an opinion, but based on my experiences working in this tech area, and in both countries.
kakao is much much more detached from the government. Its actually a pretty free thinking creative company.
but in any case, i dont think anyone should be concerned about simple sex chat on either, unless you have the misfortune to be talking to a girl that the chinese government is looking for