Zephoria offers a working definition of a social network site. 

A “social network site” is a category of websites with profiles, semi-persistent public commentary on the profile, and a traversable publicly articulated social network displayed in relation to the profile

Source: apophenia: social network sites: my definition

Some additional detail around the three attributes.  Strategic attributes usually come in threes, and more or less usually suggests the granularity is too deep or too high.  In this case, the three resonate well, and cover the broad requirements of a social site.

1. Profile. A profile includes an identifiable handle (either the person’s name or nick), information about that person (e.g. age, sex, location, interests, etc.). Most profiles also include a photograph and information about last login. Profiles have unique URLs that can be visited directly.
2. Traversable, publicly articulated social network. Participants have the ability to list other profiles as “friends” or “contacts” or some equivalent. This generates a social network graph which may be directed (“attention network” type of social network where friendship does not have to be confirmed) or undirected (where the other person must accept friendship). This articulated social network is displayed on an individual’s profile for all other users to view. Each node contains a link to the profile of the other person so that individuals can traverse the network through friends of friends of friends….
3. Semi-persistent public comments. Participants can leave comments (or testimonials, guestbook messages, etc.) on others’ profiles for everyone to see. These comments are semi-persistent in that they are not ephemeral but they may disappear over some period of time or upon removal. These comments are typically reverse-chronological in display. Because of these comments, profiles are a combination of an individuals’ self-expression and what others say about that individual.

Following a good discussion, it finishes with a nice summary.

While it’s great to talk about all of these things as part of a broader “social software” or “social media” phenomenon, there are also good reasons to have a label to address a subset of these sites that are permitting very particular practices. This allows academics, politicians, technologists, educators, and others discuss how structural shifts are prompting different kinds of behaviors.