WhatsApp is like the metric system. Used almost universally, except in the US.
I went to college in the USA, and none of my classmates had any idea what WhatsApp is.
Funnily enough, my US university's "international office" (the department that deals with international students including facilitating visas, travel signatures, SEVIS, resolving other confusions of international students etc) setup a WhatsApp group to communicate with all international students because that's the one app all of us had in common.
I mean you could just tell how unpopular WhatsApp is in the US. I still remember no one in the US have heard of Whatsapp when Facebook announce the 16B acquisition.
But WhatsUp could certainly be another Startup idea.