The 2 billion is the number of devices with the Play Store. Android devices in China do not have Google Play Store or Services on them. So devices in China do not factor into this number.
Twitter is different from facebook.Snap is not and will never be a facebook alternative(vice versa).Because social network is not just client software but relationships.You can easily replace facebook/whatsapp with snapchat or whatever you like,but how do you persuade your friends and relatives to do that?If none of your friends use the alternatives,you have to reuse facebook.
If you take a look at what wechat has done in china,you will get the answer.More and more people there are shopping,making payments via wechat pay instead of alipay,watching news through wechat news instead of other news sites,searching keywords through wechat search instead of baidu and doing almost everything in wechat.WeChat is a growing threat to other IT companies in china.In fact,alibaba has already tried several times to make a successful messaging platform but fails repeatedly(just like Google's messaging mess). If facebook move in WeChat's direction,it will be a huge threat to amazon.Please excuse for my bad English as i am not a native speaker.