I played Magic a long time ago, and I remember activated abilities on creatures were considered instants. Reading through the rules today, it isn't quite clear to me. The reason I am wondering is for the combination of Prodigal Pyromancer and Fire Servant. The does the Fire Servant's doubling apply to the Prodigal Pyromancer's ": Prodigal Pyromancer deals 1 damage to target creature or player."?
No, the doubling does not apply. Creature abilities are fast effects, a class which includes instants. But instants, like sorceries, are a distinct spell type.