According to dictionary.reference.com, a fad is quote
fadnouna temporary fashion, notion, manner of conduct, etc., especiallyone followed enthusiastically by a group.
Now, skateboarding was once a fad, but now is a trend
1.the general course or prevailing tendency; drift: trends inthe teaching of foreign languages; the trend of events.2.style or vogue: the new trend in women’s apparel.
pre·vailverb (used without object)1.to be widespread or current; exist everywhere or generally:Silence prevailed along the funeral route.2.to appear or occur as the more important or frequentfeature or element; predominate: Green tints prevail in theupholstery.3.to be or prove superior in strength, power, or influence(usually followed by over) : They prevailed over theirenemies in the battle.4.to succeed; become dominant; win out: to wish that theright side might prevail.
Now skateboarding is not a fad, because it has been around for 60 years. Meaning it is a trend. Click http://skatoramps.com/stuff/history.html to read about it.
(It basically says that skateboarding has been around a long time and has survived the speed bumps that different generations have thrown its way.)