Slate traces the history of the "you're not the customer, you're the product" line back to blue_beetle's so famous it's on our frickin t-shirts comment.
Quote Investigator looks at the history of this quotation: Richard Serra? Carlota Fay Schoolman? Steve Atkins? Tom Johnson? Andrew Lewis? blue_beetle? Tim O’Reilly?
blue_beetle gets a call-out - from the OUseful.Info blog.
Just over a year ago, blue_beetle commented that "If you are not paying for it, you're not the customer; you're the product being sold." This made the rounds for a bit, as people recognized the truth in it. (Previously and previously on MeTa) Today, his comment has been featured -- without attribution -- on a "demotivational poster" comparing Facebook users to pigs.