If you tell other people about Santa, then really you should be the one being called out. What people believe is what people believe. Whether it is arguing over the meaning of the winter solstice, or telling people a post is a prank when no one has gotten bent out of shape being fooled by it, one way or another you are attempting to ruin a holiday for people.
Have a little holiday spirit, and let people enjoy their holidays. Keep your opinions about pranks and Santa to yourselves.
Sometimes the truth, whatever it may be, is best left unknown. Ignorance is bliss. Holidays are supposed to be happy times. Don't ruin them by trying to dispel ignorance.