I really like the idea of having one single login for multiple sites and Facebook connect is a real killer application which rapidly became a de-facto standard over Internet because it simplifies our life. In short, I believe that having one single login is cool because I am myself regardless of the place where I am.
Anyway, having a single global social graph is not exactly the same thing because the friends I have for my hobbies (acquaria, ham radio, and programming) are different from my professional friends, who are in turn different from my gym friends, who are in turn different from my Italian friends, who are in turn different from my U.K. friends and so on and so forth. Also, I use other community sites powered by vbulletin and other similar software tools which encourage users to build local social graphs (see the friend list feature) for each different community site.
In this case, having multiple social graphs hosted in one central place but with different access rights for being shared would be probably much better than having one single social graph hosted in one central place.So facebook, how about being the place in internet where everyone can store his own set of social graphs and decide how, if and when they are connected?