Dive summary: 

• The Federal Trade Commission has issued letters to Google, Bing, Yahoo, and 21 other major search engines warning the companies to clearly distinguish advertising from other results.
• The letters explained that unless the firms clearly and prominently distinguished advertising from natural results they could be in violation of Section 5 of the FTC Act which protects consumers from deceptive advertising. 
• In the letter the FTC cited a survey from SEOBook in which nearly half of searchers did not recognize that top results were advertisements. 

From the article: 

"In the letter the FTC provides some concrete guidelines search engines should use to make sure consumers are not deceived by paid search results, including using stronger visual cues and using a text label that is large and visible enough for consumers to notice. The FTC recommended that shading should be more prominent with a clear outline, have a border that sets off ads from natural search results, or both."