The tendency to copy others choices when we can’t make an informed choice of our own.
Source Example 1: People were asked to watch a tea-chooser choose among two brands of Korean tea, with all packaging in Korean, so they would have no brand exposure to bias their choices. Then they were asked to select a tea for themselves. Researchers found that people were three times more likely to choose the same tea as the person they observed before they chose their teabag. Since the people could not make an informed choice about the tea since they did not understand Korean, they chose the same tea as others before them.
Sources: http://coglode.com/gems/social-default-bias, http://www.bm.ust.hk/mark/staff/YoungHuh/YoungHuh-JCR-2014.pdf
Online Example 2: Some people might not be sure if they want to vote in the next election, but these I Voted stickers might help motivate them to go out and vote. If undecided voters see a lot of people wearing these I Voted stickers, they might feel social exclusion from the primary social group, and assert that the act of voting is the social default that everyone is doing. Therefore, they might go to the nearest polling station to cast their vote to feel like they belong or feel like they made the right choice by voting.
Source: https://www.fastcodesign.com/3064600/the-surprising-genius-of-the-i-voted-sticker
Real Life Example 3: If you were new to photography, and did not know the gear you needed to get started off on the right foot, seeing what other customers bought might help you decide what else you need. The Social Default Bias not only helps Amazon customers make a purchasing decision, it helps Amazon upsell the customers into buying more stuff.
Source: Amazon.com