If you are unsure about services like Twitter, let me just say those that know it and use them often rarely post inane messages about what they are doing or just ate.
Now, Twitter or Plurk? Hmmm... Twitter has a massive number of users and a huge number or applications that work well with Twitter. Plurk has quite a few users but nothing like Twitter has and I don't know of any third party apps for Plurk.
So the answer is sign up to Twitter. However a service that does not get nearly enough attention and is as good as - if not better than - Twitter, but with significantly less third party apps, is Rejaw.
The big difference is that with Rejaw, that any message sent to it can be the start of a discussion in a thread of its own. I definitely recommend it.
Here's me:
Rejaw:
Rejaw: Ian won his Poker Tournament at The Whale!
Twitter:
Twitter: What are you doing?