I always wonder about this. I've heard some college pitchers say they never throw a fastball and others say they throw it quite often. Sam Show said she threw it regularly and that many pitchers do. Osterman I believe, said she rarely, if ever threw one.
I would imagine if you have big time velo you are more likely to keep throwing fastballs in college. Say like a Barnhill or Abbot.
And if you can put the fastball exactly where you want it (better than your other pitches), it's effective and valuable.