http://azarask.in/blog/post/socialhistoryjs/

The article has a bit of JavaScript, and a discription of what it does and how it works.

The quick summary is that it will give you a list of the social network sites that a user uses.

The most straightforward use of this thing is to put it on your blog, and have it tell you if the reader uses digg, give them a "Digg It" link, if they use Delicious, give them a Delicious bookmark link, etc etc etc

But other uses of this thing spring immediately to mind.

I, for one, wish that sites would put something like this in their signup process, and then query/scrape the other sites, and prebuild my user profile.