I don't mean to do advertisement for this APP, but if you are going to China, you need it.
WeChat, more than 600 million active users in China. You can not only chat with your friends with it, you can pay, you can book taxi, you can even order meal...It's connected to your bank account and mobile number, a kind of identification. Not so many Chinese are still using SMS any longer.
AND the most important, when facebook and google have been blocked in China, you may loss contact with your family on internet. Soooo, ask them to install WeChat too before you are leaving, just in case.