Are Sorceries and Instants considered to have Activated Abilities? Was wondering for the card {Pithing Needle}.
No: Casting a spell is not activating an ability. {Dark Impostor}'s ability is an example of an activated ability.
112.3b (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=112.3b): Activated abilities have a cost and an effect. They are written as "[Cost]: [Effect.] [Activation instructions (if any).]" A player may activate such an ability whenever he or she has priority. Doing so puts it on the stack, where it remains until it's countered, it resolves, or it otherwise leaves the stack. See rule 602, "Activating Activated Abilities."
Do Planeswalkers have activated Abilities? (Their cost being to add/subtract loyalty counters?)
Yes, that's exactly correct.
107.7. (http://imtgapp.com/forum/index.php?action=imtg;area=rule;number=107.7.): Each activated ability of a planeswalker has a loyalty symbol in its cost. Positive loyalty symbols point upward and feature a plus sign followed by a number. Negative loyalty symbols point downward and feature a minus sign followed by a number or an X. Neutral loyalty symbols don't point in either direction and feature a 0. [+N] means "Put N loyalty counters on this permanent," [-N] means "Remove N loyalty counters from this permanent," and
- means "Put zero loyalty counters on this permanent."