WhatsApp today passed 900 million monthly active users, according to founder Jan Koum. The popular mobile messaging app most recently hit the 800 million monthly active user milestone in April. Koum tagged Facebook founder Mark Zuckerberg in the announcement. (Facebook acquired WhatsApp in February 2014 for more than $16 billion.) And Zuckerberg hit right back with his own tag of the WhatsApp founder in a comment, first congratulating him and then sharing a picture of Koum at Facebook sharing the update via his phone: More information: Powered by VBProfiles VB's research team is studying web-personalization...