We're all drawn to happiness. A study at Harvard tells us that emotion is contagious. You'll notice that the Before (pre-Digital Age) pictures in Facebook's slides all display neutral faces. But the cover slide that introduces Facebook and the after slides have smiling faces on them. This is important. The placement of those graphics is an intentional persuasion technique.

Studies by psychologists show that we register smiles faster than any other expression. And when participants in a study were asked to recall expressions, they consistently remembered happy faces over neutral ones.