This is a really great summary of how people are using Twitter and why from Dan York over at Disruptive Conversations, in summary he uses Twitter:

1. as a News Source
2. as a Knowledge Network
3. as a Virtual Water Cooler
4. as a way to stay up-to-date with friends
5. as a Travelogue
6. to Track Conferences
7. as a PR/marketing Tool
8. as a Learning Tool
9. as Fun
10. as a Daily Lesson in Humility (and Brevity)

But it is worth reading the entire post.

via Aide-mémoire