Social proof is the phenomenon where people copy the actions of others in an attempt to undertake behavior in a given situation (Wiki-article).
This can be seen as a way to conform to ones surroundings. That is when one is driven by the assumption that others knows best. This accumulates, that is the majority knows best. Furthermore, how closely one is related to the crowd also influence social proof. That is one is more likely to undertake behavior given that the surrounding people are well known (family, friends etc).
Social proof is quite normal online. For example a Twitter account with many followers might seem more legit than one with few. The same goes for a twitter account a lot of ones friends are following compared to one none of ones friends are following.