If you're not paying for it; you are the product. Facebook has taken that mantra one step further than advertising with a secret study to manipulate user's feeds in order to evaluate the effect of positive or negative comments on their mood.
Of course there was outrage. No one wants to be unwittingly manipulated and many users feel the study was a breach of trust. Enter 99 Days of Freedom. Facebook users can opt in to a study on how life without Facebook impacts user happiness. Choice is a beautiful thing, isn't it?
Being a product isn't so bad as long as you're the one in charge of the emotional manipulation.