(Crossposted from the comments at Monuments.)
I've been experimenting intensely with Twitter the last month.
I find that there are two uses for Twitter (and your vision about the groups is close to that):
- What am I *doing* for my close friends.
- What am I *thinking* for a wider audience.
My wife loves having the *doing* updates, and I love hers. It keeps us close during our time apart. It is the equivalent of the SMS text messages.
The second use is far more interesting. Twitter is a tool to spread ideas - almost instantly. If you post an interesting idea/link/question/event… on Twitter, it immediately explodes. (In contrast to blogging about it - blogs have a lag-time of about a day to spread ideas.)
Now the interesting thing is (just like it is in blogging): to be able to spread ideas, you have to have an audience. And guess what, people *love* an insight in your personal live, love to get the feeling they are listening to a real person. That is why the 'what am I doing' things on Twitter ("playing with my kid") also work for the larger audience. They give a sense of personality, of realness - authenticity that is needed to make your ideas thrustworthy.
Just like in blogging, where it's not (only) about the frequency of the posts and the format and the reactions, but just as much about the personal voice.
Another thing about Twitter, again in parallel with blogging: if your motivation does not come from within, you'll give up quickly. You should Twitter in the first place because you want to write down (for yourself) the things you do and think. If you're only Twittering because you want to spread your ideas, you'll have a tough time, especially in the long run.
(Phew - such a long comment. I could've never put this in the 140 Twitter characters…)