Facebook-owned (FB) WhatsApp platform hits a new milestone with 900 million monthly active users. The tech giant's WhatsApp messaging service added 100 million new subscribers in less than five months. MAUs, or monthly active users, is a closely watched industry metric known to measure the success rate of social networking services. WhatApp has gained popularity, since Facebook acquired the app in 2014 for $19 billion. In comparison, parent company Facebook had announced back in June that it has 1.49 billion monthly active users. That was a 13 percent jump from June of 2014. The company reported that it had 1.31 billion mobile monthly active users in June, up 23 percent from the same month of the previous year. But, WhatsApp is still leading the metric versus Facebook's messenger app which currently has 700 million MAUs. After adding WhatsApp to its portfolio, Facebook has become the owner of the world's two most popular messaging apps. TheStreet's Kurumi Fukushima reports in New York.