If I don't message my sister via Facebook she won't see it. She doesn't do email for private stuff and she's not going to install another app.
If I don't message my mother via WhatsApp she's not going to get it. She doesn't understand what the fuss is and WhatsApp is already complicated for her.
I'd love to be able to message people with zero extra effort but the fact is it's a non-zero amount between us and in the end I'm the one who has to go the extra yard.
I know this because I've not had a FB account in over 7 years now. Facebook's not necessary but it sure as hell is easier.