Facebook

is a social utility that connects you with the people around you.
Facebook is made up of many networks, each based around a workplace, high school, college or geographical region.

Share information with people you know.
See what's going on with your friends.
Look up and seek people from your past, present and future
Although, facebook has an archival quality - i.e., ideas and thoughts evolving can be followed (in particular and cultural and social norms); facebook is a CURRENT expose.
"A Day in the Life of..."

Mostly teens and 20 somethings although there are many other outlying groups of members
Network systems that lead to organizational structures and thought movement

Connects users through a variety of networks (school connections, social connections, group membership)
Has complex social linking components
Integrated communication methods (messaging, wall-to-wall, announcements)
Common uses for social masses (i.e., interface is easy to use)
Visually good portal
It is an ideal way to maintain relationships with casual acquaintances whom you would not meet on a regular basis.

Very casual relationships and associations ("How do I feel today...")
However, has a very designed structure that social groups are familiar with
Growing network
popularity vs. authority
movement towards democracy
difficult to create cohesive/lasting narratives
If you are not aware of how to protect your privacy, you can be "found" by people with whom you do not wish to be in contact.
Nothing can be permanently erased on Facebook, so that the company has a vast collection of data on all of its users. Since the whole point of Facebook is to be potentially "found" by people with whom you want to be in contact, most people use their real names. This information, obviously, has many potential uses.
It invites voyeurism: you can troll through names from your past without contacting the individual people. Depending on your subject's privacy settings, you can see everything from her "friends" to her personal profile.
Facebook promotes immediate, short and, most often, unimportant communication. It is not set up for serious, longer conversations.