First, lets start with the assumption that you use more than a couple of social web apps, and each time you sign up for a new social service you find yourself wishing that all your friends were already connected to you on this new account. Enter the term "Social Graph", and the dilemna of ownership and portability. You can read in depth about it here and you can learn more about general social graph 1.0 problem here. To solve the first Social Graph problem, solutions like Facebook connect popped up. Many services let you log in via gmail or twitter to import your address books. You could register for a new service, or just log in using facebook connect, and within a few minutes you could have a your whole social graph, or at least enough to get started, connected to you. Enter Social Graph Problem 2.0:
Not only do I want to take my social graph with me, but now I want to break it down and carry that meta data with me.
Facebook, in 2006, launched a "news feed" feature dumping all your friends actions into a long stream and immediately overwhelming users across the platform. The natural reaction (besides futile protesting) was to group friends into little subgroups, to break the data down into more consumable chunks. This way Facebook could serve as a feed reader from your friends from "college", "work", or even "highschool" through their "Friends Lists" feature.
Now, I've got 600 + friends, its going to be a huge pain to do this manually. And worse, what if I want my friends categorized like this on other services? Is facebook going to let me take these lists (categories, subgroups, etc) to another service? Seesmic, Tweetdeck, and most recently Twitter have all added the ability to create lists, while services such 37 Signal's Highrise CRM sofware have powerful tagging features to categorize your contacts. Things get even more confusing when you bring in new services like, Friendfeed, Threadsy and Gist, where you can group your friends and they communications across all your services (twitter, facebook, etc)
In summary, we're pushing the envelope of data portability, and I think its exposing a bigger problem. These are my friends. These are my contacts in life, and to stay in touch with them I need a better way to organize them. These categories and lists are rapidly going to become important across all the services you use, and right now they're stuck in one place.