"Micro letter" is what automatic translation often provides as a translation, however it is a very 'non-English' construction.  Typically, people know what WeChat is, it's a product rather than a type of communication, therefore there is no need to translate the name other than to call it 'WeChat'.  As for what we send using WeChat, we might just use the word 'message', "I sent you a message on WeChat", some people will use 'text' instead of 'message', but 'message' is the most common usage, from experience.  Likewise, we would refer to WeChat as a 'messaging application' or a 'messaging app'.