There are four things that make Twitter rock.
1. Partyline: Twitter is OPEN introduction to jump into conversations. Start following some member or search by keyword there is lot of discussions with links going through twitter everyday.
2. No invite necessary, Its easier to follow on twitter. Everyone's your friend. Unless you block them. 99% users never block. For public system and someone that remembers AOL chat and IRC channels amazing how twitter design seems impervious to hazing. Hasing bad.
3. @Mentions. Dial in by throwing out question to someone by sending @ sign with their twitterID. And you get all your @mentions back where someone as dial you in one place. @mentions work on two levels. They can "wake-up" the person you are dialing but they also seen by everyone that follows you which excites lurkers. Lurking good.
4. Links. This makes it all worthwhile as open everything up to the internet.
5. Twitter augments email. Now I typically direct message to see if it is worthwhile emailing.
Twitter is still the media darling its always been been. Hands down like www, email and sms text messaging, Evan Williams and crew have best chance to develop real-time into a "utility" like the dial tone.