While the Internet has sealed the place of English as the most used (second) language, Chinese (Mandarin) still holds the top position as the most used mother tongue. In 2010, the number of Chinese native speakers totaled 955 million people. Just think of how many more people you'd be able to talk to after learning some Chinese!
Putonghua is a parlance in mainland China. It is the common language of all modern Han nationality people. In Taiwan Province and Hong Kong, it is called 'Guoyu' while in Singapore and Malaysia, it is often called 'Huayu'.