It is huge in the US also. The main feature Whatsapp enabled was free international messaging. So it immediately blew up in immigrant US populations and bigger cities where there would be more international connections.

At the time Whatsapp came out, I was in NYC, and I do not recall anyone not using WhatsApp. I remember how amazing it was to effortlessly chat with my UK family since we were no longer subject to super expensive international SMS/MMS charges.