One of the weaknesses in online advertising is whether ads are seen by actual, you know, humans. Google has now acknowledged that more than half of its ads are not on the screen for even one second. The tech giant this week released an infographic, “Five Factors of Viewability,” showing that “many display ads that are served never actually have the opportunity to be seen by a user.” The exact figure: 56.1 percent of all ads the Google and DoubleClick display ad platforms served. comScore has separately estimated that 46 percent of ads are not seen.