If you're going to refuse to join everything because of the idea that 'it's a fad', you're going to miss out on a lot of good things.
Mind you, I'm not telling you to join Facebook or join the group. Some people just don't dig their life details being cataloged online for the masses, and being owned by a corporation, as well as other perfectly good reasons.
However, the 'fad' excuse doesn't really hold water because fads are usually known as fads in hindsight (same with trends). When something has existed for almost a decade (it's going on for 9 years now with probably 7 of which being accessible to all), it's hard to see it as a fad. People at one point thought Television was a fad. The Internet, portable music, wireless networking, are other examples.