Facebook COO Sheryl Sandberg also congratulated Koum in a post saying "900 million people now use WhatsApp every month to stay in touch with family and friends around the world. Congratulations to Jan Koum and everyone at WhatsApp on reaching this incredible milestone -- and for still finding time to make us all laugh."
source :- http://timesofindia.indiatimes.com/tech/tech-news/WhatsApps-global-user-base-crosses-900-million/articleshow/48807248.cms