Daily life in cities tends to differ from daily life in small towns, especially by who we interact with. The MIT Senseable City Lab and the Santa Fe Institute studied this social aspect — individuals' contacts by city size — through anonymized mobile phone logs. As expected, those in cities with greater populations tended to have more contacts. However, when the researchers looked at who knew who, the results were more constant.
Surprisingly, however, group clustering (the odds that your friends mutually know one another) does not change with city size. It seems that even in large cities we tend to build tightly knit communities, or 'villages,' around ourselves. There is an important difference, though: if in a real village our connections might simply be defined by proximity, in a large city we can elect a community based on any number of factors, from affinity to interest to sexual preference.
Read the full paper for more details.