One of the major psychological foundations for the purported effectiveness of social media marketing is social proof. The idea is fairly simple: people are more likely to find a product or service appealing if there is evidence that other people find it desirable. Robert B. Cialdini, in his very popular Influence: Science and Practice, describes social proof as
an important means that people use to decide what to believe or how to act in a situation by looking at what other people are believing or doing.
We’re social animals, and signals from our peers as to the desirability of particular objects has a significant influence on whether we want them too. In social networking, this principle is put into practice through the pursuit of “likes” or “+1s”, and through an active pursuit of conspicuous evidence of engagement and approval for products and brands among consumers.