One of my ongoing projects is my address book file. Currently, it's just a text file, but it's in a regularized format, and I will eventually write a script to convert it into vCard format, and into FOAF format.
One rich source of digestible content for the address book file are social networks. I've been pulling in data from LJ, MySpace, Facebook, LinkedIn, Flickr, and LibraryThing.
I've been noticing that different social network have different cultures and norms for contact information, intersection to the realworld, and intersection with other social sites.
LJ is decent for "other site intersection". People often fill in email addresses, and their userids for AIM, YM, ICQ, and Gtalk. Most of those IDs map to profiles on those systems, which can be mined as well. But getting a real name out of an LJ profile is hit or miss. People almost never fill that form out correctly, using it instead as an avenue for some sort of catchphrase or personal artistic expression.
LJ fits very closely with my own brain's way of remembering people, by their narrative. That's probably why it's my own current preferred social site.
Getting anything out of MySpace is Hard. It is the worst of the silos, it never wants to let you out, and so has very few structured methods for having outbound links. Even a person's flist has almost no useful information. MySpace users collect "friends" the way that grade school kids collect Pokemon cards, warez dudz collect cracked games, and crazy old ladies collect sick cats.
The basic technology of, the manipulation by the corporate owner of, and the evolving cultural norms of MySpace have created a near perfect storm of transient identity, simple narrative, and adolescent expression, with all the reality and depth of Hot Topic.
Facebook is very nearly the opposite. It doesn't allow pseudonyms, and binds hard to the user's realworld identity. For most users, their realworld name is right there in their profile. That the signup process and the process of friending someone involves a roundtrip and authorization to a hardware token (specifically, a cellphone), and that being "friended" requires confirmation by the target strongly signals and enforces that cultural norm.
LinkedIn is also at that high level of realworld identity binding. It's been fun wandering around LinkedIn, finding old coworkers, and adding them to my network pool.
Flickr and LibraryThing are interesting cases. Instead of having a do-everything silo as their business model, they instead focused on one particular passion that some people share (photography and books, respectively). They built their social network infrastructure because they needed to, not because they wanted to.
They are very much like LiveJournal in a way, just like I can learn a lot about someone by reading their narrative, one can learn a lot by looking a person's library, or photo collection.
As the various solutions to the distributed social graph get off the ground, more and more single passion sites like them will bloom, because the expensive barrier of harnessing interpersonal network effects to drive growth will be much lower, as the social network is a resource to be drawn on, rather than an annoying and expensive database technology and marketing outreach problem to build yet again.