Bandwagon effect
The bandwagon effect is a propaganda technique that suggests one should do something because everyone else is doing it. It is an argumentation fallacy, because something does not have to be true if everyone is doing it. It comes from the idea of a parade, where happy people go by on bandwagons and people in the crowd have the urge to 'hop on'. The bandwagon effect is an age-old technique used to recruit soldiers for war or sell hamburgers to the masses.
This McDonald's ad is a good example of the bandwagon effect. We see happy people walking by and eating at the restaurant. If we want to be as happy as the people in the ad, then we had better eat at McDonald's.