Andrew tipped me off to iminta.com, a social network aggregator. It's the first I've tried, so I can't really compare it to anything else.
Anyways, you tell it what sites you're on; It natively talks to twitter, flickr, youtube, google reader, digg, delicious ... a bunch of others. It's *very* easy to set up. And once set, it tracks your activity on all of your social network sites.
Then your friends sign up, supplying the same infos.
All your social networking sites and all of their social networking sites stream a continuous flood of data into the tubes, and iminta happily aggregates it. Then it emails you a daily update, telling you all about how your cow orkers wasted their whole day on twitter and digg.
After a couple days the novelty wore off for me; too much information without any filtering. But I'll recommend it *heartily* to the data-addicted!