The Collaborative International Dictionary of English v.0.48:

Fad \Fad\ (f[a^]d), n. [Cf. Faddle.]
   1. A hobby; freak; whim.
      [1913 Webster]

            It is your favorite fad to draw plans. --G. Eliot.
      [1913 Webster]

   2. a practise followed enthusiastically by a number of people
      for a limited period of time; as, the latest fad in
      fashion.

   Syn: craze; mania.
        [PJC] -- Fad"dist, n.
        -- Fad"dish, a.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

faddist \faddist\ n.
   a person who subscribes to a variety of fads.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

faddist
    n 1: a person who subscribes to a variety of fads

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "faddist":
   addict, bug, collector, craze, demon, eager beaver, energumen,
   enthusiast, fad, faddiness, faddishness, faddism, fanatic, fiend,
   freak, great one for, hobbyist, hound, infatuate, novelty, nut,
   pursuer, rage, rhapsodist, sucker for, visionary, wrinkle,
   zealot