A suitable way to explain the difference (there is!!) between a plain hobby and a fad is to refer to the site www.crazyfads.com. On their subject, fads, they say:
"From 1920 to 1990 we list all of the craziest fads that have come and gone. Go back a few decades and read about the silly to serious fads that helped change our society and create a pop-culture.
ant to know which fads your parents experienced? What about the ones you experienced yourself? Maybe you will start a new fad yourself after reading some of the crazy fads at crazyfads.com.

Go see for yourself!