On Valentine’s Day, Nick Bergus came across a link to an odd product on Amazon.com: a 55-gallon barrel of ... personal lubricant.

He found it irresistibly funny and, as one does in this age of instant sharing, he posted the link on Facebook, adding a comment: “For Valentine’s Day. And every day. For the rest of your life.”

Within days, friends of Mr. Bergus started seeing his post among the ads on Facebook pages, with his name and smiling mug shot. Facebook — or rather, one of its algorithms — had seen his post as an endorsement and transformed it into an advertisement, paid for by Amazon.

In Facebook parlance, it was a sponsored story, a potentially lucrative tool that turns a Facebook user’s affinity for something into an ad delivered to his friends.