It's official, Pownce has been bought by Six Apart and will promptly be shut down by the middle of this month. Pownce was another clone of the Twitter microblogging social network that had a few more extra collaborative features, but it just wasn't better or different enough to justify most people moving over.
I used Pownce (http://pownce.com/rubenerd/) for a period of time earlier this year, but only because my desktop Twitter client at the time also allowed you to simultaneously post to it. I thought the interface was a bit cleaner and more interesting, but at that point I already had several hundred followers on Twitter as well as all my friends. When I moved over to TweetDeck for my desktop Twittering (I've changed Twitter clients, again I ceased updating my Pownce profile.
For what it's worth, I was never quite comfortable with the idea of having all my shared media part of a closed or gated site like this; it's the same reason I don't put anything of value or substance on my Facebook profile either. A Twitter approach which keeps the underlying messaging infrastructure simple with links you post yourself to media you want to share makes much more sense, and is infinitely more flexible. Brightkite (which I reviewed back in October) is a perfect example of this kind of "ecosystem" approach where multiple sites and services are used together to create something wonderful.
I wonder how Jaiku is going under Google these days? I had a Jaiku profile (http://rubenerd.jaiku.com) which also used to be updated with the same Twitter desktop client software I used in the past too, but since moving to TweetDeck which only posts to Twitter I let that wither as well. It makes you wonder whether Identi.ca stands any chance at all.