Facebook began inserting advertisements directly into users' news feeds

                      There won't be a lot of ads--just one per day--but they will be unsolicited. Instead of being on the right hand side of the page and marked "sponsored," they will appear randomly within users' news feeds and be labeled "featured."

                       According to Facebook's Help Center, "Businesses can pay to feature a post so there's a better chance you'll notice it." As an example, the Help Center writes, "Say you like your gym's Facebook Page. Some friends see the story in their news feeds, others may miss it. The owner of the gym can pay to feature the story so your friends are more likely to see it."
This is pretty much the way the "sponsored" stories work, but those are much easier to distinguish since they're not embedded in the news feed.