Popular mobile messaging service WhatsApp has crossed 600 million monthly active users, CEO Jan Koum announced. The company had crossed 500 million monthly active users in April, which means it has added 100 million users in just four months. WhatsApp is clearly leading the pack followed by Tencent’s WeChat with 438 million active users, and Viber with 100 million active users. Koum also takes a dig at other messaging services who do not reveal the number of monthly active users, saying, “Yes, active and registered are very different types of numbers.”