Twitter's recent and downtime have left a hole in the nanoblog market, to the extent that such a market actually exists. Among the bloggerati, FriendFeed is filling in the vaccuum and could become the new Twitter. It's got a good feedback system and it also has features that make finding and adding friends very easy. And FriendFeed reads in Twitter content, so users can have the best of both worlds.
Now there's an even newer Twitter clone: Plurk (about the name). Its big advantage is its user interface. You get a slick timeline view of all the posts from your friends. The system also has a good design for adding friends by finding them on your other buddy lists (AIM, Yahoo IM, Gmail, etc.). Also, there's a good method for responding to Plurk posts that shows you a conversation thread laid on top of the timeline. You can also post videos, photos, and links easily. And there's a decent grouping function for segmenting out sections of your friend list.
It is a great interface for a nanoblog service, although it works best on a full-screen browser. There is a nicely done mobile version at plurk.com/m, but it doesn't give you all the visual candy.
Unfortunately, Plurk has a bit of Twitteritis: It's not the most reliable service on the planet. An influx of users over the weekend (which is being blamed on or credited to Leo Laporte) has apparently overloaded the system, and occasionally users may find elements of it not working.
The bigger issue with a service like this, though, is the network of users. Yes, Plurk looks great and has very strong features. So do Jaiku and Pownce. But Twitter is where people hang out, at least for now, assuming Twitter can fix its reliability issues.
Also, by focusing on the user interface to such a great extent and ahead of building an API, Plurk discourages developers from building their own clients (like Twhirl for Twitter), some of which could make the service more attractive to important subsets of users. (Although, to be fair, separation of interface and service hasn't done much for Twitter.)
Plurk is a creative nanoblog service but feels a bit redundant in the Twitter/FriendFeed era. There is the chance it could open up the nanoblog concept to a new group of users not on the current platforms, though.