Social proof
Social proof is the phenomenon where our behavior is influenced by the behavior of others. Figure 31-1 shows a classic example of this principle in use online where the opinions of others are linked to products and services to encourage others to purchase.
Liking in Facebook (and on the wider web) is a modern manifestation of social proof that represents a specific piece of functionality that came from Facebook. This feature can be added to any website and allows people to add a like thus endorsing that particular content object. Figure 31-2 shows an example of how this is often used, note how the total number of people who have also liked an object is shown to reinforce its popularity (or unpopularity).