Few are immune to the trend. It brings you closer to fifth-grade friends, peers and even parents, for better or worse. Xanga, MySpace, Friendster, YouTube, Twitter and Facebook — what is the science that makes social networking such a staple of 21st century life?

As Prof. Michael Macy, sociology, stated in an e-mail, “A plausible case can be made that our distant ancestor’s dependence on the clan shaped the evolution of a strong tendency to identify with a group, and this ‘we’ feeling in turn seems to have been an important part of the attraction to Facebook, at least initially. thefacebook.com, as it was originally known, started out at Harvard, and then expanded to other Ivy schools, which gave Facebook an exclusive and elite identity. Feelings of strong attachment and loyalty to one’s college — and college friends — may reflect in part a strong human tendency to identify with an in-group.”

The number of Facebook users has exploded since its 2004 launch. This begs the question: Where do all those Facebook friends (read friend-of-a-friend you met at a party that one time) play into all of this? Humans are, after all, social creatures.

British anthropologist Robin Dunbar is known for calculating what is known as “Dunbar’s number” — a measure that limits the number of people that an individual can realistically maintain relationships with. That number is 150. Take a look at your friends list — chances are that it easily exceeds Dunbar’s limit.

That surplus is a sign of the times. Facebook is a very real extension of our constantly expanding everyday world, so it’s no surprise that we have a growing extended network to socialize with. However, it is likely that many of these social-networking-friends are, in fact, “real world” acquaintances — placing our average closer to Dunbar’s 150.

Macy said he hopes social scientists harness social networking technology for their research. “We now have the ability to study patterns of human social interaction on a global scale,” he said. “I also find it remarkable how these global networks make it possible for people to find others with similar interests, no matter how idiosyncratic (or even ‘deviant’) and this may help to preserve and encourage cultural diversity in the face of pressures toward cultural standardization and homogenization,”

From an evolutionary stand point, the ability to empathize with particular actions quickly (for example fear in the faces of our kin) was a central factor in our brain development.

However, more subtle emotions still often require face-to-face interaction for full comprehension. Similarly, Linda Stone coined the term “continuous partial attention” to refer to modern lifestyles that require individuals to keep track of multiple inputs both voluntarily and involuntarily.

This stimulus overload could eventually create a form of social numbness and empathy, like the kind perhaps responsible for the passive friend relationships ubiquitous on Facebook networks.

One way or another, it seems that Facebook is just in our nature.