It really depends. Social networks cover a wide net of media sources from tumblr, twitter, facebook, linkedin and even comicvine. All are products of phatic technologies and essentially are a product of human need. They make things easier for a lot of people. Like people with large families or kids that go abroad for school.
Useful. It would be incredibly difficult to stay connected with anyone post-highschool without Facebook. Its an archive on a fairly large portion of your life, through photos, memories, and shared moments.