Facebook recently began to show sponsored stories in the site’s main news feed and in its mobile apps, where they appear a lot less like traditional ads, though they do bear a “Sponsored” label. It has told investors that consumers were 50 percent more likely to recall an ad if it came with a plug from a Facebook friend. And it has made clear to users that while they can change a privacy setting so their “likes” do not appear under ads in the most prominent advertising zone on Facebook pages, they cannot turn off other kinds of endorsements that show up elsewhere. “Because sponsored stories are just stories from the news feed, you cannot opt out of them,” Facebook explains in its help center.