I've always wondered if sites do this the other way around. A site invests a lot in content they create you would think to get the most out of it they would serve it with different headlines to different demographics. Knowing from Facebook what headlines you have clicked through in the past can indicate how they should write future headlines to get you again.
Oh, they definitely do it the other way around extensively.
In the testing phase, what happens when you click the headline?
Sometimes it just loads a random article, sometimes it gives an error page.