Theoretically this is true. It is even true in practice. But…
I think you underestimate the level of annoyance and inconvenience it causes. Not to the non-Facebook (or whatever) user but to the people who do use it and don’t see any reason why someone else wouldn’t.
There is a limit to how often they are prepared to go to the extra effort of emailing the one person who doesn’t do Facebook or to phone them or whatever.
Your description is quite right - not using Facebook or whatever is (was?) terribly gauche. It is not the fashion. It inconveniences others, thus making it actively impolite not to use it.
Whether we like that or not, it was (is?) true.
My mother, to take a random example, is certainly capable of emailing and using the telephone but she bugs me about why I’m not on whatsapp - it would be so much more convenient.
Even though she can’t explain how it would be more convenient 
But everyone else she knows and is in contact with uses whatsapp so she does and so should I.
