Messenger and WhatsApp are popular in different regions but combine both and you have the lions share of worldwide mobile chat users. For example, Messenger is

Messenger and WhatsApp are popular in different regions but combine both and you have the lions share of worldwide mobile chat users. For example, Messenger is much more dominant in the US, while WhatsApp is crazy popular in places like Brazil or Germany. So i guess they are just starting with the US market (Messenger) and will eventually offer similar stuff on WhatsApp.