I just Plurked a little while ago. No, I'm not ill - Plurking is the act of posting a message on Plurk
(weird name, I know!). It seems to be like Twittering, except that it doesn't have the mobile (maybe they do and I just haven't found it yet?) or IM elements to it - you have to be at a web browser to use it.
Their timeline interface is quite nice and is their unique selling point. I wonder how long it'll keep them ahead though - there are so many browser add-ons, desktop applications and web front-ends for Twitter that somebody could just come up with a Plurk-like interface for Twitter.
Other non-core functionality on Plurk is the Karma that you accumulate for activities (Eg. when someone 'follows' you, you earn karma. When someone 'leaves' you, you lose karma.).
I wonder why we need another social networking site though - we've got so many already. I just join the ones that I know where most of my friends are - FaceBook and Orkut!