For example, I was an avid user of IRC back in the late '90s. That medium formed my social experience and those people became many of my life-long friends. These days many of my friends from that time (including myself) don't use social networks much at all; Facebook, Instagram, and Twitter included. We've been there, done that, and are finding human engagement without social networks to be more satisfying. Consequently, my contemporary relationships are not significantly maintained through social networks and whether someone is or is not on Facebook or Instagram has no bearing on whether my relationships are maintained.
A wise friend of mine once said "Facebook is for people you don't know!"