And if you are categorizing things into 'productive' and 'non productive', what kind of net benefit to society do you think Zynga produces?
http://dictionary.reference.com/browse/parasite
a person who receives support, advantage, or the like, from another or others without giving any useful or proper return, as one who lives on the hospitality of others.
(in ancient Greece) a person who received free meals in return for amusing or impudent conversation, flattering remarks, etc.
Perhaps you are right as the definition of parasite in regards to people generally indicates living off others by their consent. Rather than living off others by threat or use of force.
Zynga produces entertainment.