"Social graph" is horrible web 2.0 speak.
Here's an ugly diagram:
It's a solid concept though, and it's what we build on Facebook, Twitter, Linkedin and every other social networking site we all use.
Each of these sites brings its own particular slant and flavour to the type of social graph we construct:
Facebook - focuses on real life friends / acquaintance
Twitter - more nebulous, but some focus on online communities / clustering around key online tastemakers
Linkedin - professional networking
Google/GMail - ?
As pointed out here:
We only tend to be 'friends' with a small proportion of our Google contacts. And everyone uses email very differently. Do I really want my recruitment consultant to see my photos from a night clubbing?
It's such a generalised, and in online-terms, longstanding communications format that everyone uses it in different ways. This means that far from having a 'ready made' network on Google Buzz, most users will need to make serious efforts to tune their audience appropriately.
Even given a willingness to do this, there is still no obvious way to connect with the people you want to see your 'Buzz'.
So although I wouldn't go so far as to predict a failure from Google here (*cough* knols, orkut), I'm confident that it'll take a lot of hard work and tweaks and marketing to get this to take off as effectively as they'd like.
The notion google are pushing that it taps in to your already existent 'Google' social network is simply not true.