Well, thanks mainly to Steve's bad influence, I have recently joined many different social networks (and tools) and I thought I'd just give you my opinion on a few of them.
Swurl : Swurl offers you the ability to bring all your web content into one place using feeds. It's great for fun things like my feed timeline but I find it's not so good at following your friends feeds.
Socialthing! : This is a utility whereby you can bring together all of your friends status' from different sites (Facebook, Twitter, Plurk etc). This makes it easier to follow your friends, but currently it's invite only. Drop me an email with your email address if you want an invite!! I admit to quite liking Socialthing, even if it's not the most attractive site in the world.
Brightkite : It's invite only too I'm afraid, again send me your email address if you want one! This app allows you to check in at your location. Basically you are logging your physical location as a status. You can then comment on it too, add photos etc. Early days yet... I think it could be amazing if I knew more people who were using it.
HelloTxt : This application allows you to microblog from multiple locations and keep track of other status' ala Socialthing! This makes it rather better than Ping.fm although Ping.fm is better for cross posting on blogs, IMO.
Identi.ca : An opensource competitor to the currently troubled Twitter. Watch this one, I think it'll become quite cool if it takes off properly.
Also rediscovering the joys of Last.fm.