Social proof is the influence that the actions and attitudes of the people around us (either in real life or online) have on our own behavior. The "proof" element is the idea that if other people are doing it (or saying it), it must be correct.
The degree to which social proof affects us can be a result of the numbers of people seeming to promote something or the particular individuals involved. The actions and opinions of social influencers carry more weight with their colleagues, social network and the general public than is the case with most other individuals. Social media influence is most often associated with online marketing but can also refer to the way social networks and influencers affect the behavior of users in other areas, such as politics.
Social proof can influence people to take actions and make decisions differently than they would if they had relied solely upon their own judgment. The undue influence of others can result in conformity and errors. Social proof is demonstrated in mob behavior and the lack of individual opinion sometimes referred to as herd mentality.
According to theories around social proof, the question, "If all your friends jumped off the bridge, would you?" is generally answered in the affirmative.
Watch Danielle Stephen's TED talk on social proof: