Back when I used AOL/AIM in high school there was an explosive adoption of "always on" internet services such as DSL and cable. During this time, it became fashionable for teens to express their current emotions/actions/likes as their "Away Messages" on AIM, which was something new compared with the limitations their previous dial-up connections. I'd go as far to say these messages were the precursor to services like LiveJournal.

Since college, with the mass adoption of social networks, I would guess that 95% of all my friends no longer use independent chat services, like AIM.

I see twitter as the next iteration of "Away Messages" - or, what they now call status updates.

Personally, I dislike Twitter for my personal use. I think it is a fascinating way for brands to sell products to unsuspecting consumers, but there is no value as a standalone "Away Message" platform... that would be the job of something like Facebook status updates, since all of my friends are already connected on there.

As for how it grew? It both filled the need for people to continually express themselves and that other really important thing... celebrity endorsements.