Metafilter user blue_beetle first put this idea online when he said "If you are not paying for it, you're not the customer; you're the product being sold" in response to the Digg revolt of 2010. The idea apparently existed for a few decades prior regarding TV advertising. I prefer to think blue_beetle was the one who brought it into the zeitgeist.
http://www.metafilter.com/95152/Userdriven-discontent
http://quoteinvestigator.com/2017/07/16/product/
Edit: Alex3917 posted a similar idea on HN on 6 May 2010, beating blue_beetle by a couple months. Gotta give credit where it's due: https://news.ycombinator.com/item?id=15030959