These grandpeople had to have been using something else 10 years ago to communicate with their grandkids... and then over these 10 years then simultaneously learned how to use FB, forgot how to use that other technology they'd used their whole life, and also lost the ability to learn how to use any other communication technology?
Not impossible with cognitive deterioration, but probably a lot more rare than this excuse.
If my grandparents were still alive today I'd just tell them that Facebook was controlled by the Democrats and we should use X,Y, or Z instead of effectively donating to Obama's campaign. They'd have deleted the app and gone back to text messages and phone calls by the end of dinner.
This blows my mind. I mean having Whatsapp implies one has a phone which implies one can receive SMS messages or God forbid, a phone call.