Where in Sweden have you not found WhatsApp common? Or what age group? In Sthlm and nearby towns (ages 25-45) all I ever hear is WhatsApp and Facebook Messenger. Since I don’t use any of those people roll their eyes, sigh and reluctantly agree to SMS, since they don’t use any proper messaging apps.
I’m in Stockholm as well, no one outside ‘expat communities’ has attempted to communicate with me using WhatsApp.
Who knows though, maybe I’m not in tune with the general public any more. I know that basically all of my extended family run on iMessage, and some friends use Facebook Messenger as their lowest common denominator.
To be fair I’m not really in tune with the general public either, but every time I’ve had to link up with someone, whether old friends, classmates or co-workers they’ve always gone for WhatsApp immediately. I think iMessage is very popular, too, but I’ve never heard people mention it by name. Probably because it’s just the default iPhone-only SMS replacement.
Just to provide some perspective on the Nordics claim, since I’m Norwegian and worked in a phone store for six years. In my experience, practically no one here uses Whatsapp, except for communicating with people in other countries. It’s either Facebook messenger, Snapchat or SMS/iMessage.