Facebook has begun rolling out an update to the Graph Search algorithm, which now includes posts that have been shared with you in the results. That means if a friend shared a post with you, like a news item or a photo, they will appear along with other related searches that you make.
The update, for the moment, applies only to those who have their language set to US English on the desktop web version, and in the Facebook for iPhone app which will receive an update sometime this week.
The update comes after feedback from Facebook users who stated that they also wanted results from shared posts. For a more detailed explanation on how the algorithm works you can visit this page. Graph Search was announced in June 2013 and started rolling out for everyone a month later.