I recently signed up to Identi.ca (around Monday evening GMT).
Basically it’s like Twitter but it’s Open-Source, and has a large following from the Ubuntu community, a lot of the members from the Forums are on there too 
As well as being open-source, it supports OpenID which means you can use 1 login across multiple websites (read the link for more info).
In all I actually prefer it to Twitter (I deleted my Twitter account). You can find me at http://identi.ca/Joeb454
