Pownce pwns me.
Back in March, I signed up for Twitter after months of hearing about it ... and have hardly touched it.
I got an invite for Pownce after hearing about it on digg for a couple weeks ... and have hardly been anywhere else on the web in the three days since. digg? Sure, I stop in, once or twice a day -- instead of thrice an hour. Local news, national news, tech news, just about everything I refresh regularly has been cut waaaaaaaaaaaaaaay back.
What is Pownce? It's definitely a social network ... microblogging oriented ... some aspects of presence management or indication perhaps ... easy file sharing (post videos, pics, MP3s, etc. etc. on your blog, no HTML required, no external host required) ... interesting threading of conversations -- blog-like, forum-like, IM-like, all rolled into one.
I'm currently out of invites, and have a waiting list for fresh invites, but if you're interested, drop a line, I'll add you to the list.