If you want to live life by what your family thinks of you, by all means, do so. Respectfully, however, if you can't consider what strangers on the internet have to say, you're significantly limiting its usefulness to you.

I'll turn that hypothetical question you posed around and ask how saying that email isn't sufficient for keeping in touch is any different from saying "Who needs greeting cards? Just let Facebook tell you to post on someone's timeline when it's a birthday or holiday."

I'm not denying that Facebook has some utility, but it's changed our definition of keeping in touch to value mass quantity over quality. If one values quality communication, Facebook is completely unnecessary and can be replaced with email which is an existing, standardized, ubiquitous, and sufficient means of keeping in contact with people. Your family would be statistical outliers if almost all of them didn't have an email you could send to.

In conjunction with "your life depends on facebook?", I'm simply saying that while one can use Facebook, there is such a thing as a viable alternative with its own advantages. It's controversial, I know.