Facebook Tip: Starting in August, Facebook started mixing older stories (‘most relevant’) in with new stories in feeds. So, if you have your feed set to “Sort: Most recent”, you may be seeing older posts.

“Facebook announced a couple small changes to the way that the site decides order in News Feed: Story Bumping, which allows engaging posts you haven’t seen to be bumped up to the top of News Feed later in the day, and Last Actor, which takes into account the last 50 engagements a user has performed, and gives those users a slight bump up in News Feed ranking.”

More here:
http://allfacebook.com/facebook-announces-story-bumping-and-other-small-changes-to-news-feed-algorithm_b123088

(Facebook is also penalizing folks who post a lot of “low quality” which includes memes).