The oft told story of social proof is that a person walks up to building with empty restaurants on both sides. That person picks restaurant A, A second person walks up and sees one person in restaurant A and no one in restaurant B. that person chooses A, and so on. Restaurant A is a major success, Restaurant B must re-concept the space because it has fails on a massive scale.
There is a also the dynamic that social networks be definition build much the same way. People who would never have joined on their own are forced to join because everyone else is in the network. Individuals are very uncomfortable when they are not part of a common whole.
The business lesson is that the key is to create a social network that individuals wish to be a part of in your business.