By linking to a 55 gallon drum of water-based lubricant on Amazon from his Facebook page, he became Amazon's pitchman for said lubricant to his friends.

Amused, I posted it to Facebook with the line “A 55-gallon drum of lube on Amazon. For Valentine’s Day. And every day. For the rest of your life.” And then I went on with my life.
 A week later, a friend posts a screen capture and tells me that my post has been showing up next to his news feed as a sponsored story, meaning Amazon is paying Facebook to highlight my link to a giant tub of personal lubricant.
 Other people start reporting that they’re seeing it, too. A fellow roller derby referee. A former employee of a magazine I still write for. My co-worker’s wife. They’re not seeing just once, but regularly. Said one friend: “It has shown up as one on mine every single time I log in.”

In this day and age, it's good to keep in mind that if the service or web site you're using isn't charging you anything, there's a good chance you are the product.

Via kottke.org